Page:EB1911 - Volume 21.djvu/739

Rh nematocysts develop in the Turbellarian is doubtful, and it is not impossible that they are derived from the tissues of some coelenterate animals eaten by the Planarian, as has been shown to be the case in the nematocysts of Eolids. (3) Cells producing aciculate spicules, sometimes associated with a spiral thread. These structures are often associated together in batteries, notably so in the remarkable genus Anonymus.

Excretory System.—The excretory system consists of protonephridia, that is, of tubes opening to the exterior by one or more apertures, and after branching extensively in the mesenchyma, end blindly in peculiar hollow cells (flame-cells) provided with a bunch of synchronously vibrating cilia. The excretory tubules have a markedly sinuous course and are provided with cilia. The motion of these cilia and of the flame-cells is to induce an outward current of the fluid from the canals, but the process of excretion seems to be performed chiefly by the branched mesenchymatous flame-cells. The position of the external opening varies greatly. It may be single or paired, mid ventral or terminal, or again multiple and arranged in pairs along the dorsal surface (Tricladida and probably in Polycladida). The flame-cells are arranged in pairs in Tricladida, but lie less regularly in the mesenchyma of most forms. Finally, it is noteworthy that in the Acoela no excretory system is known.

Nervous System.—The nervous system is present in all divisions of the order. It consists of a paired, anterior ganglion lying ventral to the gut, and from this are given off, right and left, dorsal, lateral and ventral fibres interconnected by a plexus. The nerve-cells are scattered throughout the plexus. The chief development of the system occurs in relation to the muscular body-wall, sense-organs and the pharynx. In these characters the nervous system of Planarians shows an interesting transition from the scattered plexus of Coelenterates to the segmental ganglia and sympathetic nervous system of Annelids.

Sense-organs.—These occur in the form of tactile organs, otocysts and eyes. The whole skin of many Planarians is sensitive, and amongst the ordinary locomotor cilia long stiff ones are found which it is natural to think are tactile organs. The head-end is often provided with specialized cells that appear to subserve the sense of touch and possibly of taste also. The abundance of rhammites, of long stiff cilia, and the great mobility and sensitiveness of this region, bear out this conclusion. A further development of cephalic, sensory structures occurs in the form of a crescentic groove (Polyclads) of paired, lateral pits (Microstoma, fig. 10) of mobile papillae on the extreme front margin (Land Triclads) and of extensible tentacles, marginal or nuchal in most Polyclads.

The otocyst occurs constantly in the Acoela and sporadically in every other division of the group. It is with one exception a single median organ placed over the brain, and consists of a uni- or bi-cellular sac containing a calcareous

concretion lying in a fluid. From what is known of these organs in higher invertebrates we may infer that they serve to increase the perception of slow wave-movement and enhance the control of the muscular sense.

Eyes are generally present in Planarians. Two types are distinguishable—eyes with a cup-shaped retina facing outwards, and those with an inverted retina facing inwards. The former occur in Triclads and Polyclads around the margin of the body often a hundred or more may be present. The latter occur in all groups except the Acoela, but are limited to the neighbourhood of the brain or bases of the nuchal tentacles. Recent investigation has shown that the essential part of the eyes has in all cases a complicated structure and is not a mere epidermal cell-group enclosed by pigment and provided with an optic nerve. On the contrary (Hesse [10]), adequately known eyes are composed of rod-cells that contain each an axial filament or bundle of fibrillae (the termination of the nerve), and the distal end of the rod-cell is converted into a striated usually broad border where the action of light commences. A group of such specialized rod-cells is enclosed in a pigmented cup opening either outwards or inwards and pierced by an optic nerve. The whole is usually depressed beneath the epidermis, but in some Acoela and Alloeocoela the eyes retain a surface-position. In the Polyclads eyes may increase by division and in Triclads may decrease in number by fusion (Carrière [11]). The marginal and often radial disposition of the scattered eyes, and the prostomial position of the paired eyes, afford interesting evidence of the intermediate position that Planarians occupy between the radiate Coelentera and the bilateral Annelids.

Reproduction.—All Turbellaria are hermaphrodite, and reproduce sexually, but a few forms (Microstomidae and some Triclads) increase during the summer by fission and during the winter by eggs. The body of the Microstomidae becomes constricted and partially subdivided into two, the posterior half regenerates a brain and pharynx. Subsequently each becomes again converted into two zooids, and the process is repeated until a chain is formed as in fig. 10. This breaks up into its constituent members, each of which repeats the process until the onset of reproduction. The