392 MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. 



pairs pass forward on either side of the masticatory stomach to a point about on a level with the 

 lirst maxillary segment. The ventral is the larger and louger of these, and two lobules are con- 

 stricted off from it near its extremity. They correspond to the ventral lobes of the midgut {mg.^ 

 cut, Fig. 2). The dorsal pair represent the anterior lobes (mg.'), which are now entirely withdrawn 

 from the head region, and naturally contain uo food yolk. The gastric cffica are all tilled with a 

 coagnlable fluid which stains feebly in carmine. The gastric epithelium for a short distance behind 

 tiie point where glands communicate with the stomach has marked histological peculiarities. The 

 internal absorbent surface is increased by folds which nearly obscure the lumen of the tube. The 

 cells are columnar and resemble the glandular cells of the liver and probably have the same origin 

 as the latter. In the masticatory stomach there is a strainer of hairs developed on the ventral 

 and lateral walls which are greatly thickened, as we saw in the larva. The dorsal wall is thin, 

 but there is a large valvular fold on the ventral side. 



The vascular system of the adult is already outlined in the larva in all its essential character- 

 istics. Tlie walls of the blood vessels are exceedingly delicate, so that it is not easy to ascertain 

 tiieir distribution by means of sections alone. The heart (PI. liii, Fig, 190, H.) is a short tubular 

 cliamber, flattened between the dorsal body wall and the enlarged section of the hind gut. It is 

 suspended in the pericardial sinus (j?. s.) to the body wall and surrounding organs by means of 

 strands of connective tissue (alte-cordis). The walls of the heart are quite thin, and its cavity is 

 l)artially divided into three compartments by the growth downward from its roof of two sheets of 

 mesoderm cells (PI. lvi. Fig. 231, and PI. li. Fig. 186). 



Of the several arteries which lead from the heart, three, and possibly five, can be distinguished. 

 Posteriorly the heart is continuous with the large superior abdominal artery, which traverses the 

 abdomen close to the dorsal wall of the intestine (Figs. 196, 2.32, 235, a. s. a.). Near its origin 

 from the heart, the sternal artery (Fig. 196 shows a trace of this vessel between ganglia 12 and 1.3, 

 to the left ofpr.) is given off, and passes directly downward to the ventral nervous system, which it 

 penetrates at a point between the third and fourth thoracic ganglia. This is continued backward 

 under the nervous system and forms the inferior abdominal artery (Figs. 229-234, a. i. a.) Anteri- 

 orly the heart gives off the unpaired ophthaJmic artery (Figs. 196, 215-229, op, a. op.), which runs for- 

 ward to the region of the eyes and brain. It is not an oi)hthalmic artery, strictly speaking, but from 

 the first, supplies arterial blood to the brain and anterior cephalic region generally. In Figs. 215, 

 210, it is seen cut in partial longitudinal section, where it evidently communicates with the blood 

 space surrounding this part of the brain. The antennal arteries can not be clearly distinguished 

 in sections, but in a much earlier stage trains of cells are seen at-the surface of the egg passing 

 forward on either side of the middle line toward the eye stalks, which jiossibly represent the anten- 

 nal vessels. 



Besides the sinuses already mentioned, there is a large sternal sinus (Fig. 196, sts. s.). This 

 occupies the extensive space between the thoracic ganglia and the alimentary tract and " liver," 

 and, like all other similar spaces, is more or less completely tilled with serum and blood corpuscles. 



Five pairs of gills are present at this stage. They are developed from simple pouches or folds 

 of the skin on the bases of the thoracic appeiidages (Figs. 193, 230-233, hr '' ^), The outer sur- 

 face of this primary fold soon becomes divided into a number of secondary folds or gill plates, and 

 in a larva which has moulted twice and is twenty-four hours old, the branchia has the structure 

 shown in Fig. 195. The adult gill is precisely similar to this, except that it has a greater number of 

 plates and more definite branchial vessels. In the early larval stages the skin and especially the 

 branchiostegites (Fig. 193, bg.) probably serve as important respiratory organs. 



In respect to its muscular system the first larva ai»pears to differ but little from the adull. 

 Tbe flexor and extensor mu.scles of the abdomen are most prominent (Fig. 196, nni. /., mii. r.). The 

 former consists of a double rope of fibers, fuse 1 completely together and very much twisted. They 

 extend from the sides of the thorax to the termiual telson (Fig. 227-235. mu.f.). The extensor 

 muscles [mu. e.) are smaller, but otherwise similar to the latter, both in origin and extent. They 

 lie above or to the sides of the digestive tract. Their attachment to the carapace is shown in 

 Figs. 227, 228. 



The next most prominent muscles are the adducitors of the mandibles and great chelae. The 

 former consists of a large band of fibers which pass from one side of the body to the other directly 



