MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 391 



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

 This in tlie larva corresponds to the midgut (Fig. 190, mg.) and its diverticula. 



The oesophagus (Figs. 196, 218-220) is a straight, vertical tube, with very thick walls, which 

 are throwu into longitudinal folds. There is an anterior and posterior fold and two lateral ones, 

 which give to the lumen of the oesophagus the shape of the hitter X when seen in transverse 

 section (PI. LVII, Figs. 241, 242). The walls of the masticatory stomach resemble those of the 

 oesophagus, and the folds of the latter are continuous with the valvular structures of this region. 

 The lateral and median thickenings (PI. LV, Fig. 221, p. v.) at the point where this portion of 

 the stomach passes into the midgiit may be regarded as a rudimentary pyloric valve. The pouches 

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

 gastrolith sacs in the crayfish embryo (54), but no gastroliths are found in Alplieus. 



The midgut appears in the longitudinal section (Fig. IOC, mg.) as a short, lestricted cavity. 

 It is, however, a spacious chamber, as we see by examining a series of sections made in other planes 

 (Pis. LV-LVII). 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 (mg.*), a pair of posterior (mg.' J ) 

 and a pair of ventral lobes (mg. 2 ). All these parts are lined with a peculiar columnar epithelium, 

 composed of endoderm 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 

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

 since also the eudoderin is formed very gradually and first appears in the region where the hind 

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

 masticatory stomach (of ectodermal origin) and that of the midgut (Fig. 19G) is most pronounced. 

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

 from communication with the yolk until very late in embryonic life (PI. XLVIII, Fig. 168). The 

 anterior lobes contain the remnant of unabsorbed yolk (Figs. 218, 237, ?/.), and in cases where the 

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

 rated by a median vertical partition (mp.), composed of connective tissue and muscle cells, which 

 suspend this portion 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 anterior lobes is 

 incomplete. The dorsal half of it consists of a downward-growing fold of endoderm cells, with a 

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

 that the eudoderm 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 P. 1 LI, Fig. 185, mg."). They lie to one side 

 of and below the hindgut (PI. LVI, Figs. 226-230, mg\, #<;.' J ). 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 in 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, </</.') is constricted off' by the growth of a fold from the 

 side next to the hindgut, downwards and outwards, to form a secondary lobule (gg. 2 ). 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 best 

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

 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. 2 ) are ventro-lateral diverticula from the central portion of the 

 midgnt and are completely lined with columnar epithelium. 



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

 lighten the anatomy of the 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 consist of three pairs of lobes or ca;ca. One pair, corresponding 

 to the posterior division of the midgut (Fig. 22(>, mg. :! ), is imperfectly divided into three lobules, 

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



