Page:Proceedings of the Royal Society of London Vol 60.djvu/58

50 Fig. 7.—Optical section of larva of L. reticulum, first day, x 500. difficult to follow, b u t in botb im m igration appears to take place from any point on the surface.

In L. cerebrum and L. reticulum th e larva swims for about tw enty-four hours at th e surface, and as long at th e bottom, and fixes on the th ird day. L. coriacea, on th e other hand, is rem arkable for its abbreviated larval period as compared w ith the two M editerranean species, since the larva fixes in a few hours, a fact doubtless in connexion w ith its life betw een tide m arks, where the violent currents to which it is exposed renders a very sheltered, and therefore limited, habitat necessary for so delicate an organism.

After fixation, the larva undergoes changes w hereby the ciliated cells become surrounded by th e form erly internal granular cells, so th a t the ciliated external layer of the larva represents th e gastral layer of the adult, while the inner mass becomes the dermal la y e r; the reverse of w hat was supposed by Metschnikoff and Schm idt (loc. cit.) to take place.

In L. cerebrum I was able to observe the first appearance of the spicules. As in varibls, the complete metamorphosis results in a stage in which the gastral cells form a compact internal mass, sur-