MEMOIKS OF THE NATIO^IAL ACADEMY OF SCIENCES. 391 



with endodermal epitlielimn near the point where it communicates with the cavity of the gut. 

 This iu tlie larva correspond.s to the midgut (Fig. 196, mg.) and its diverticula. 



The (I'sopliagus (Figs. 106, 218-220) is a straight, vertical tube, with very thick walls, which 

 are thrown into longitudinal folils. There is an anterior and posterior fold and two lateral ones, 

 which give to the lumen of the resophagus the shape of the letter X when seen in transverse 

 section (I'l. LVir, Figs. 241, 242). The walls of the masticatory stomach reseml)]e those of the 

 (esophagus, and the folds of the latter nrv continuous with the vah uhir structures of this region. 

 The lateral and median thickenings (PI. i.v, Fig. 221,2*. r.) at the point where this jiortion of 

 the stomach passes into the iiiidgnt may be regarded as a rudirnentai-y ])ylori<! valve. The |>ouches 

 formed between the median ventral fold (Fig. 221) and the lateral folds (;). r.) correspond to the 

 gastrolith sacs in the craylish embryo (o4), but no gastrolitlis are found in Alpheus. 



The midgut appears in the longitudinal section (Fig. 19G, w[/.) as a short, iestri<;ted cavity. 

 It is, however, a siiacious chamber, as we see by examining a series of se<;tions made in other ]ilanes 

 (Pis. LV-i.VU). It consists of seven parts or divisions: a dorsal, unpaired, median division {mg. 

 in all the figures), and, opening from this, a pair of anterior lobes (»ig.^), a pair of posterior (mg.') 

 and a pair of ventral lobes {mg.''). All the.se parts are lined with a peculiar columnar ei)ithelium, 

 composed of endodenn cells, derived primarily from the wandering cells, excepting a part of the 

 median and the anterior divisions, where the endodermal wall is absent or only imperfectly formed. 

 The epithelium of the midgut passes imperceptibly into that of the intestine, since the cavity of the 

 hindgut is in communication with the food yolk from the very early stages of the embryo, and 

 since also the endoderm is formed very gradually and first api)cars in the region where the hind 

 gut communicates with the yolk. On the other hand, the demarcation between the wall of the 

 inasti<!atory stomach (of ectoderuial origin) and that of the midgut (Fig. l!t(i) is most pronounced. 

 Correlated with this distinction is the fact that the foregut is a blind sac and completely cut ott 

 from communication witli the yolk until very late in embryonic life (PI. XLViii, Fig. 168). Tlie 

 anterior lobes contain the remnant of unahsorbed yolk (Figs. 218, 2.37, _(/.), and in cases wliere the 

 lining epitlielium is unformed, the food yolk is in contact with the brain. These lobes are sepa- 

 rated by a median vertical i)artition {mp.), composed of connective tissue and muscle cells, which 

 suspend this i)ortion of the digestive tract to the anterior dorsal wall of the body. In a very late 

 embryo which is about ready to hatch we find that the partition separating the anlerior lobes is 

 incomplete. The dorsal half of it consists of a <lown ward-growing fold of endoderm cell.s, with a 

 mesodermic core. Tlie ventral and lateral walls of these diverticula are devoid of epithelium, so 

 that the endoderu) extends itself most rapidly forward, on the dorsal median line, and thence spreads 

 to the ventral floor. 



The posterior lobes {mg.'') are the first to develop (see PI Li, Fig. 185, mg.''). They lie to one side 

 of and below the hindgut (PI. LVi, Figs. 226-2.30, «!//'., gg.^'-'). Up to this stage their position is never 

 dorsal to other parts of the digestive tract. It is from these lobes that the gastric gland or so called 

 " liver" arises. Each lobe is simple until a short time before the embryo hatches, but iu the newly 

 born larva it is divided into three lobules. This division is effected in this manner: The lower 

 median part of the primary lobe (Fig. 228, gg.') is constricted ott' by the growth of a fold from the 

 side next to the hindgut, downwards and outwards, to form a secondary lobule {gg.^). By the 

 constriction of the upper portion in the same way the primary lobe becomes divided into three 

 pockets. The relations of the posterior division of the midgut to the unpaired central portion is be.st 

 shown in a horizontal section (Figs. 236-238). It seems quite probable that a part of the ei)ithelial 

 lining belonging to the enlarged section of the hindgut is endodermal in its origin, but just how 

 much it is impossible to say. 



The ventral lobes (Fig. 224, mg.^) are ventro-lateral diverticula from the central portion of the 

 midgut anil are completely lined with columnar epithelium. 



An examination of the structure of a young Alpheus of this species, ten days old, throws much 

 light on tiio anatomy of tlio larva Just considered. The alimentary tract has at this time essen 

 tially its adult structure. The gastric glands open into it by short ducts at a point just behind 

 the masticatory stomach. They consi.st of three pairs of lobes or cieca. One pair, corresponding 

 to the posterior division of the midgut (Fig. 226, mg.'), is imperfectly divided into tiirce lobules, 

 as in the early larva. They extend backward, below and to one side of the gut. The two remaining 



