MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 417 



The mesenterou (mg.) is very largely reduced in size and is tilled with a granular coaguluni, 

 and, anteriorly next the head, with vesiculated aud eroded remnants of yolk. The endoderm lines 

 all but the anterior third of this cavity, extending farther forward on the dorsal wall. A few 

 wandering cells are still present in the peripheral parts of the cavity next the advancing edge of the 

 eudoderiu. Those elements, represented in Fig. 182, prove to be eudoderm cells mechanically 

 detached from the wall of the meseuteron. The primary lobes of the midgut ("liver") are larger 

 but otherwise similar to those described in Stage x. The endoderm cells are greatly vesiculated, 

 aud the cell protoplasm has often a striated appearance. 



The heart (Figs. 180, 180, H.) has undergone very considerable changes since the period repre- 

 sented by Figs. 104, 168. It is no longer dorso -ventrally flattened, but in transverse section it is 

 triangular in appearance. Oue side of the triangle is toward the intestine aud one apex next the 

 body wall. Its suspension in the pericardium is very delicate. The ectoderm cells send down 

 spindle-shaped processes (Fig. 186), the Ectoderm pfeiler of Keichenbach, aud to these, meso- 

 dermal elements become attached. The cavity of the heart is imperfectly divided by lateral 

 partitions into three longitudinal compartments. In Fig. 186 the partitions are imperfect and 

 represented on each side by a single rudimentary muscle fiber. The walls aud partitions of the 

 heart are composed of delicate muscle fibers, which are distinctly striated. In the abdominal 

 muscles, striations can also be made out. 



STAGE XII. THE FIRST LARVA. 



We now reach the stage with which this paper began, the first larva of Alpheus saulcyi. 

 The histological structure of the zoea in the species with a regular metamorphosis differs only in 

 minor particulars from the larva already described. The organs are all very much smaller, and 

 the cells are relatively larger and less compact. The mesenterou is about half tilled with the 

 unaltered and uuabsorbed food yolk. Wandering cells are almost entirely absent, aud the endo- 

 dermal walls are nearly complete. The partition between the masticatory stomach and the midgut 

 is absorbed and communicatiou between them is established. 



The anterior and median lateral divisions of the midgut are present, but the posterior lateral 

 lobes are represented only by spaces not as yet walled in by endoderm. There is a slight dorsal 

 median fold of endodermal cells. In the larva of the same species three days' old the posterior 

 lateral lobes are formed, but are very small. 



STAGE XIII. ALPHEUS TEN DAYS OLD. 



In the first twenty-four hours the larva moults twice, but the histological changes in this 

 period are not of a very extensive character. The organs which experience the most rapid growth 

 are the gills (PI. LIII, Fig. 195.). These have now acquired the folds or plates for increasing the 

 respiratory surface, and are more efficient as breathing organs. 



The fibrous tissue of the brain is relatively greater in bulk, and the tracts of fibers are more 

 numerous and more complicated. The eye stalks are much shorter, aud the optic ganglia and 

 anterior parts of the brain are drawn closer together. 



In a larva four days old (PI. xxi, Fig. 3) the eyes are completely covered by the carapace. 

 The ganglia of the eye stalks aud brain are intimately fused together. The nervous system and 

 all the tissues have undergone greater or less histological changes. These can be more conven- 

 iently considered in a still older larva. 



The period of metamorphosis, strictly speaking, is passed in about twenty-four hours after 

 the time of hatching. The structure of an Alpheus ten days old, which had spent its entire life 

 in an aquarium will now be briefly considered. It is sexually immature aud some of the organs, 

 like the "liver" aud green gland, are less complicated, but otherwise the structure is essentially 

 that of the adult form. 



When we compare the brain of the first larva with that of the ten days old and the adult 

 fully grown form, we find the same parts present in all. In the last two the fibrous tissue is rela- 

 tively much greater in bulk, and ditt'erentiation of the fibers and fibrous tracts has advanced much 

 farther. The brain cousists of the same fibrous masses surrounded with a thinuer cortex of nerve 

 and ganglion cells. 



S. Mis. 94 27 



