Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/693

Rh CRUSTACEA 659 Mr Adam White Ceratocephalus Grayanus, offers many points of analogy with the extinct Trilobites. The fragile mode of articulation of its walking legs, and their entire FIG. 72. Ceratocephalus Orayanus; Flinders island, Bass s Straits. A, the dorsal aspect. B, front view of the head showing the curvature of the three spines. C, side view. D, ventral aspect : g, the glabellal spine ; I, I, the two lateral or genal spines of the head; e, the eye; a, the antennule ; a&quot;, the antennas, the legs are seen folded up beneath the body. E, the mouth enlarged, showing ep, the epistoma or upper lip plate, TO, the mandibles, m&quot;, the )iectinated maxilla, a, the antenna; the epistoma is set in the base of the glabellal spine g. . concealment beneath the body-segments, are very suggestive. This Isopod is near to the Sphaeromidve, but will form a dis tinct family, as the antennae are inserted beneath and within the margin of the head-shield ; apparently it does not roll itself into a ball. The Sphceromidae are very littoral in their habits; they range from the equatorial latitudes to the colder temperate zones, but are not found in Polar regions. They are veget able feeders, and some (like Limnoria) are guilty of destroy ing timber. When molested or alarmed they roll themselves up into a ball. The Sphceromidce present many points of analogy, if not of affinity, with the extinct Trilobites. In this order we find the Limnoria terebrans (or the &quot; gribble,&quot; as it is called by the fishermen). It is one of the most destructive creatures, attacking all woodwork below tidemarks ; the only wood which it cannot destroy is teak. Although its ravages had gone on for ages, it was only made known to the scientific world and described by Dr Leach in 1811. In the aberrant genus Tanais (fig. 38) the first pair of thoracic legs are converted into chelae, and the head-shield is covered by a carapace, abundantly traversed by currents of blood, beneath which a stream of water passes, main tained as in the zoeae and adult Decapoda by a flabellifonn appendage of the second pair of maxillae, which is wanting in all other Edriophthalmia. The abdominal feet, which in other Isopoda act as respiratory organs, are simple natatory feet in Tanais. These characters, together with the pedunculated eyes and the great chelate hands, give to Tanais a very decapod-like aspect (see fig. 38). The Idoteidce contain representatives of some of the largest known Isopoda, some of which are above 4 inches in length. The JEgidix!^ and C ymothoidce have all the feet furnished with a robust finger, sharp at the tip, for seizing and holding on to fishes upon which they are parasitic. Another family, the Bopyridce (fig. 39) are parasitic chiefly on members of their own class, frequently occupying the branchial chamber of the common prawn, and distorting the carapace 1 One species, Eurydice pulchra, common in the Dee, Cheshire, actually attacks bathers. &quot; If you remain a moment still in the water dozens will fasten on you and nip most unpleasantly. I have had to jump into the water again after coming out from bathing, and splash violently to get rid of the hosts that had stuck to me while clinging to the side of the boat preparatory to getting in. They con tinue to bite after you are out of the water.&quot; Extract of letter from Mr Walker to Mr C. Spence Bate. on one side. A fossil Bopyrus is observed lying in the branchial cavity of a crab (Palceocorystes) from the Gault and Greensand. 2 We have already referred to these and other parasitic forms in the earlier part of this article. Isopoda are met with as far back as the Old Red Sand stone, where remains of a gigantic species allied to Arcturus have been discovered ; others occur in the Carboniferous and Oolitic periods. The Purbeck beds of Swauage, Dorset, also yield abundance of a freshwater form, the Archaeoniscus Brodiei ; species of Oniscus and Sphceroma are found fossil in the Cretaceous, the Eocene, and the Miocene of Europe. II. EDRIOPHTHALMIA : (4.) TRILOBITA. So long ago as 1821 Audouin placed the Trilobites with the Isopoda, whilst Macleay assigned them a distinct order between the Isopoda and the Phyllopoda. Later researches by Milne-Edwards Fin. 73 1, Leperditia Baltica, Wahl. ; U. Silurian, Gothland. 2, Entomoconctiut Scouleri, M.C. ; Carbonifs. L, Ireland. 3, Beyrickia comp/icata, Salter ; Lr. Silurian, Wales. 4, Dithyrocaris Scouleri, M Coy; Carbonifs. L.Ireland. &amp;lt;&amp;gt;,I tery~ gotus anglicus, Ag. ; Old Red, Forfarshire. 6, I restmctua rotundata, II. Woodw. ; Carbonifs., Coalbrookdale. 7, llloenus Davisii, Salter, L. Silurian, Bala. 8, Phacops caudatus, Brunn. ; U. Silurian, Dudley. 9, Calymene M/umenbachii, Br. ; U. Silurian, Dudley. 10, Trinucleus ornatus, Sternb. ; L. Silurian, Britain. 11, Agnostus trinodus, Salter; Lr. Silurian, Britain. and others have caused the Trilobita to be referred to the Entomostraca, on account of the very variable number of body-rings observed in the several genera (from six to twenty-six) evidencing a much lower type of structure than the Isopoda, in which the thorax is composed of seven free and movable segments with a head-shield and anchylosed caudal somites. Moreover, until the discovery of pre sumed ambulatory appendages in an Asaphus from the Trenton Limestone in 1870, 3 the only appendage previously observed was the hypostome or lip-plate. There seems, however, no good reason to urge against the conclusion that the Trilobita were an earlier and more generalized type of Crustacea, from which the later and more specialized Isopoda have arisen, a view which the writer is glad to say he shares with the distinguished carcinologists, Professor Dana 4 in America, and Mr C. Spence Bate in England, although at present more evidence is needed as to the nature of the locomotory appendages in this extinct group. If we range the characters of Trilobita and Isopoda side by 2 Mr James Carter, F.G.S., lately showed the writer & Palceocorystes from the Cambridge Greensand, having a Bopyrus lodged in each of its branchial chambers. 3 Billings, Quart. Journ. Geol. Soc., vol. xxvi. p. 479. 4 Professor Dana writes, &quot;The Trilobita probably belong with this second type &quot; (the Edriophthalmia, or Tetradecapoda, as Dana names them) &quot;rather than with the Entomostraca. Yet they show an aberrant character in two important points. First, the segments of the body are multiplied much beyond the normal number, as in the Phyllopoda among the Entomostraca ; and Agassiz has remarked upon this as evidence of that larval analogy which characterizes in many cases the earlier forms of animal life. In the second place, the size of the body far transcends the ordinary Isopodan limit. This might be considered a mark of superiority ; but it is more probably the reverse. It is an enlargement beyond the normal and most effective size, due to the same principle of vegetative growth which accords with the (occasional) inordinate multiplication of the segments in the body&quot; Journ, Science, July, 1856, vol. xxii. p. 11),