MICROSCOPICAL STUDIES. 65 



cocoons are transparent, gelatinous, and tough, and composed of 

 several layers, enclosing a large number of eggs — each egg in turn 

 encased in a transparent spherical membrane. 



The course of development of the embryos is abbreviated, for 

 when they free themselves from the egg-membranes, they possess 

 the general structure of the adult. Indeed, as soon as born, they 

 swim and dart about with all the confidence of full-grown indivi- 

 duals. Even the ink bag is developed at the time of hatching, and 

 is to be seen as a tiny black spot, and upon irritation, the little crea- 

 tures can cause the contents to be ejected in a tiny black cloud. 

 There are also present chromatophores — very conspicuous vesicles 

 of pigment, governed by sets of muscles that produce by alternate 

 expansion and contraction the pretty blushings and pallor so marked 

 a feature among the adult Cephalopods. I noticed, however, that 

 the chromatophores are normally kept in a state of contraction so 

 long as the animal is unhatched, the whole mass being glassy trans- 

 parent till then. For a long time before freedom is gained, the 

 embryos can move freely in their capsules. In these young forms, 

 a tiny remnant of the yolk sac, attached in the centre of the arms, is 

 sometimes unabsorbed at the moment of birth. Two flap-like fins 

 are present at the extremity of the abdomen — and of very different 

 Bhape to the adult fins. 



The young of Sepia have a similar history, but here the eggs are 

 laid in separate egg capsules and not in cocoons. Black and of the 

 form of grapes, these eggs produce each a single embryo. 



In the mounted specimens of the embryo, notice the pair of gills at 

 the hinder part of the mantle cavity, a number characteristic of all 

 living Cephalopods excepting the pearly Nautilus which possesses 

 four gills. On this account we class all Cephalopods as either 

 DlBRANCHS or TETRABRANCHS. The present day representatives of 

 the class are grouped as follows : — 



Class :— CEPHALOPODA. 

 ORDER I :— TETRABRANCHIATA. One living genus only ;— 



Nautilus. 



Order II :— DIBRANCHIATA. 



Sub-Order I : — Octopoda (possessing eight arms), whereof the 

 best known genera are Argonauta, Octopus, and Eledone. 



Sub-Order II : — Decapoda (possessing ten arms), containing a 

 very large number of genera ; among others being Sepia, 

 Sepiola, Loligo, Rossia, Ommatostrephes, Loligopsis, and 

 Spirula. 



Explanation of Pl. VII. Figs. A. B. & C. 



Fig. A. A cluster of egg capsules of Loliga Forbesii, attached to a 

 rope, x t>. 



