[11] DEVELOPMENT OF THE OYSTER. 789 



blood-corpuscles of the oyster are also probably not developed in appre- 

 ciable numbers until after the true larval stage is past. 



The intestine of the larva is a simple internally ciliated tube; indeed 

 the entire alimentary canal appears to be ciliated from the mouth to the 

 vent, and, as in the adult, there do not appear to be anything like un- 

 striped annular muscular tissues developed around the intestine so as to 

 produce anything comparable to the peristaltic action of the intestine 

 observed in worms, arthropods, and vertebrates ; the food is carried into 

 digestive tract and the excrements out of it by the action of the cilia 

 alone. 



The intestine of the spat long before the stage here figured has been 

 reached already contains food or its indigestible remains, but the longi- 

 tudinal fold found in the intestine of the oyster, as well as in the intes- 

 tines of many other lamellibranks, is but feebly developed in the hind 

 gut of young oyster spat. It is, however, present as a pretty well 

 marked ventral ridge or slight induplicature of the intestinal wall, but 

 it is evidently not clearly folded inward upon itself, as in the adult, 

 until the animal is much older than the stages studied by the writer. 



The retractor muscles of the velum in the larva do not appear to be 

 homologous with any of the muscles of the spat or the adult, and are 

 analogous only to the radial muscular bundles r of Fig. 2. These 

 radiate from around and near the insertions of the great posterior ad- 

 ductor w, and are most strongly developed in the posterior and ventral 

 halves of the mantle lobes (in Fig. 2 these radiating pallial muscles are 

 perhaps too strongly indicated about the hinge-border of the young 

 animal). The retractors of the velum appear to traverse the blastocoel 

 in the embryo. The pallial retractors, on the other hand, are embedded 

 in the connective tissue of the mantle next to the outer epithelial cover- 

 ing or epidermis. The radiating pallial muscles become more complex 

 with advancing age, and as the adult condition is approximated, and 

 while there is a decided thickening of the margin of the mantle in the 

 spat there is not as strong a development of the marginal muscle as in 

 the adult. In the adult the radiating pallial muscular bundles also re- 

 peatedly divide as they j^ass towards the margin of the mantle, a trait 

 which they possess to a much less marked degree in the spat. The 

 radiating and marginal muscles have their fibers very much interlaced, 

 so that a very complex arrangement of the muscular fibers is finally de- 

 veloped at the edge of the mantle. 



The fringe of the tentacles along the border of the mantle of the spat 

 is also much less strongly developed than in the adult. They are, in 

 fact, in the young spat mere papillary elevations, as shown in Fig. 2, 

 and at first there appears to be only a single row of them, whereas there 

 are two rows in the fully grown animal. As the animal increases in 

 size these marginal tentacles of the mantle also increase greatly in 

 length until they become finger-like in form, with a usually more or less 

 well-marked purplish coloration throughout their epidermis and about 



