DEVELOPMENT OF THE SQUID. 9 



It is interesting to notice that there are other groups of Mollusca, among which the 

 mantle cavity is produced in more than one way. In Cyclas, Pisidium, and many other 

 Lamellibranchs, a fold grows out from the body, and thus converts the space between 

 the fold and the body into a mantle cavity precisely as in Loligo, but in Anodonta the 

 shells are quite well advanced in their development before the mantle cavity is formed, 

 and when this is produced it is formed by the retraction of the body wall into the space 

 between the shells ; and we have in Anodonta almost precisely the same modification of 

 the primitive history which Grenacher describes in the Cephalopoda. In the oyster agaiu, 

 we have the mantle cavity produced by a process which is about midway between that 

 met with in Cyclas, and that presented by Anodonta. 



Projecting from under the overhanging edge of the mantle, on the posterior surface of 

 the body, at the stage shown in figs. 9 and 10, are the rudimentary gills, g, one on each 

 side of the middle line. They are formed as Httle papillae, or outgrowths from the 

 surface of the body, and are covered with cilia. In position and method of formation 

 they are very much like a single pair of the gill-tentacles of a Lamellibranch embryo, 

 and their embryonic history would seem to indicate that they are to be regarded as 

 greatly specialized gill filaments, rather than structures comparable to the entire gUl of a 

 Gasteropod or Lamellibranch. 



The shell area in the middle of the mantle is now a deep pit, fig. 9, s, and its edges 

 have begun to fold towards each other, as shown in fig. 10, s. ks, Lankester has 

 pointed out, it originates in exactly the same manner as the shell-gland of a Gasteropod 

 or Lamellibranch, and is strictly homologous with these structures. 



The eye stalks, es, figs. 9 and 10, are now very conspicuous projections from the 

 sides of the dorsal end of the embryo, and the eye-invaginations are well developed, 

 and their external openings much smaller than the inner portion of the invagiaated 

 sac. In fig. 10, the left eye is shown in a surface view, and the right eye in profile. 

 The lateral folds of the siphon, fig. 10, si', are a little more definite than they were 

 during the previous stage, and the two inner siphon-folds, fig. 9, si, have made their 

 appearance on the posterior surface of the body. They are not only separate from each 

 other, but widely separated at this time from the lateral folds. Opposite the outer 

 ends of the inner siphon-folds the ears, fig. 9, er, have mdae their appearance, as two 

 almost spherical invaginations of the integument, communicating with the exterior by 

 large openings. Nearly opposite the inner folds of the siphon, on the median line of the 

 anterior surface of the body, the mouth, mo, fig. 10, is now visible as a blind sac with 

 a large opening pointing downwards. 



On each side of the mouth, there is a very faintly-marked imdulating line or 

 fold of the surface of the body, «, which runs from a point near the corner of 

 the mouth, out into the eye-stalk, crossing the outer ventral edge of the eye. The 

 line makes two well-marked dorsal undulations, and one ventral one, the latter near 

 the middle of the line. Any small particles which are floating in the vicinity of this line 

 are thrown into active motion in the direction of the arrows, thus showing the presence of 

 cilia too small to be visible by any magnifying power which can be used. The position of 

 this line, its relation to the mouth and eye-stalk, and the presence of cilia along it, all 

 indicate that it is to be regarded as a rudimentary velum. 



