12 W. K. BROOKS ON THE 



cavity is very shallow, and hardly extends beyond the edge of the mantle^. In both 

 Loligo and Sepia, the mantle is much smaller, the chromatophores are wanting, and the 

 mantle-cavity extends nearly or quite to the dorsal extremity of the body. 



The fins of Sepia and of Grenacher's embryos make their appearance at about the 

 same time, and are developed later than they are in Loligo. 



The fact that three such closely related forms, belonging to the same group of 

 Cephalopods, differ from each other in the order of development of all organs of the body, 

 except the otocysts, is a striking comment upon the assumption that the order of 

 development of parts, as observed in a single species, can give any information as to their 

 morphological importance, or as to the order in which they were acquired in the evolution 

 of the group. We must notice, too, that the series which these three forms furnish 

 when any one organ is taken as a basis of comparison, may be quite different from that 

 given by another organ, and it is clear, without comment, that we have no information at 

 present which will allow us to generalize upon such questions as to which of these forms 

 recapitulates the phylogenetic record most perfectly. There are a number of interesting 

 points, however, in which all three agree. In all, the halves of the siphon are separate at 

 first, and all pass through a stage in which the siphon resembles that of a Tetrabranch. In 

 all, the eyes are at first open pits, like those of the Nautilus, and in all they are at some 

 time carried upon long stalks. In all there is a time when only four of the five pairs of 

 arms are present. 



Passing now to the next stage in the development of Loligo, plate 11, fig. 13, is a 

 view of the posterior surface of an embryo somewhat older than in fig. 12. The external 

 yolk sac, y, has grown so much larger that only a small part of it is shown in this 

 and the next three figures. The mantle, m, has grown so much that the gills, g, and the 

 rectum are nearly contained in the mantle-cavity. A constriction across the base of 

 each gill has separated the branchial heart, h, from the gill proper. The inner folds, si, 

 of the siphon, have united with each other to form the closed siphon tube, and the inner 

 and outer folds, si, si', have met and are uniting with each other. 



The walls of the otocysts, er, have grown thin, and their cavities have greatly enlarged ; 

 the otoliths have made their appearance, and the two chambers have begun to move 

 towards the median line, under the end of the siphon. 



The external openings of the otocysts have become constricted to long, tortuous, 

 ciliated ducts, which are not visible with a low power, and are not shown in the figure. 



The eye-stalks, es, are of about the same relative length as in the last figure, but the 

 yolk prominences which have filled them up to this time are now almost entirely with- 

 drawn or assimilated, and the cavity of the eye-stalk is nearly filled by the ball of the 

 eye, e, the optic ganglion, ga, and the white body, to&. 



The arms have lengthened, and suckers have appeared upon the longest pair, a", and a 

 new pair, a', have made their appearance upon the posterior or siphonal surface of the 

 body. In Grenacher's embryo this pair of arms make their appearance as buds upon the 

 next pair, a", but in Loligo they are distinct from the first, and make their appearance as 

 elevations of the integument upon the surface of the body. 



The yolk is now divided into four well-marked regions, the external yolk sac, y', 

 which is still nearly spherical ; the head yolk, y", which is pretty nearly cylindrical, and 



