MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 417 



The inesenteron {mg.) is very largely reduced in size and is filled with a granular coaguluni, 

 and, anteriorly nest the head, with vesiculated and eroded reniiiants of yolk. The endoderni 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 [)arts of the cavity next the advancing edge of the 

 endoderni. Those elements, represented in Fig. 18:i, prove to he endoderm cells mechanically 

 detachetl from the wall of the luesenteron. The i»riuiary lobes of the midgut ("liver") are larger 

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

 and the cell protoplasm has often a striated appearance. 



The heart (Figs. ISO, ISti, H.) has undergone \ery considerable changes since the period repre- 

 sented by Pigs. 1G4, 168. It is no longer dorso ventral ly flattened, but in transverse section it is 

 triangular in appearance. One side of the triangle is toward the intestine and one apex next the 

 body wall. Irs suspension in the [lericardium is very delicate. The ectoderm cells send down 

 spindle-shaped processes (Fig. 186), the Ectoderm p/eiler of Keicheubach, and to these, meso- 

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

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

 represented on each side by a single rudimentary muscle fiber. The walls and 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 zoiia in the species with a regular metamorphosis differs only in 

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

 the cells are relatiNtjly larger and less compact. The meseuteron is about half tilled with the 

 unaltered and unabsorbed food yolk. Wandering cells are almost entirely absent, and the eudo- 

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

 is absorbed and communication 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 (I'l. liii. Fig. 195.). These have now acquired the folds or plates for increasing the 

 respiratory surface, and are more eHicient 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, and the ojitic 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 and 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 alter 

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

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

 like the "liver" and 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 ditlerentiation of the fibers and fibrous tracts has advanced much 

 farther. The brain consists of the same fibrous masses surrounded with a thinner cortex of nerve 

 and ganglion cells. 



S. Mis. 94 27 



