BULLETIN OF THE UNITED STATES FISH COMMISSION. 1G3 



a simple unpaired rudiment, in opposition to the observations of Lacaze- 

 Duthiers, according to whom both halves of the shell originate, "par 

 deux boursouiiements de Penveloppe", (?) which afterwards unite to 

 form the hinge. Brooks, in discussing this point recently ill regard to 

 the American oyster, observes that the shell from the first consists of 

 two distinct halves, which develop froin a small, irregular, transparent 

 tract which lies in and athwart the dorsal groove or depression— his 

 blastopore. 



If it is also borne in mind, as I have before observed, that the pecu- 

 liar character of this groove and the true blastopore have apparently 

 escaped the observation of the last-named author, then we may be 

 justly allowed to entertain some doubts as to the correctness of his in- 

 terpretation. On the other hand, the description given by Hatschek of 

 the first appearance of the shell in Teredo, agrees perfectly with that 

 observed by me in Ostrea, and we may, as it appears to me, with safety 

 assume that the development of the shell in all mollusks takes place in the 

 name icay. This admits of no question; and, as the last-named investi- 

 gator very justly observes, it is a weighty argument in support of the 

 position so ably defended by Yon Jhering, viz, the theory of the mono- 

 phyletic descent of the Mollusca. 



Meanwhile, the ectoderm frees itself over almost the whole circumfer- 

 ence of the embryo from the entoderm, so that now, for the first time, a 

 body cavity (segmentation cavity) becomes apparent; a crown of cilia 

 is also developed above the mouth, and the velar area (included by the 

 ciliary girdle) is composed of columnar cells (Fig. 9). The entoderm has, 

 meanwhile, enlarged and includes a spacious stomach cavity, from which 

 a sac-like outgrowth is developed below, which still ends blindly, but 

 which will afterwards be fused with the ectoderm to form the anal end 

 of the intestine. 



In the stage of development attained by the next day (Fig. 10), the 

 shell has grown considerably in size. It now covers a large portion of 

 the body, and, as indicated by treatment with acids, is already in part 

 composed of lime carbonate. After the application of dilute acetic acid 

 there remained only a tough membraue of conchioline. The ectoderm 

 2ells, which lie below the shell, have by this time become extremely flat- 

 tened and transparent, so that one can no longer make out their con- 

 tours, and with difficulty their highly refringent nuclei. The larva (Fig. 

 11), which now 7 takes in nourishment, moves about with a lively motion 

 and begins to grow slowly ; the velum now forms a prominent portion 

 of the body, which will be entirely covered by the shell as the latter 

 grows larger. The velar area, which is included by the ciliary crown of 

 the velum, is already slightly thickened in the center, the rudiment of 

 the velar plate. A funnel-shaped oesophagus passes into the wide pear- 

 shaped stomach which communicates posteriorly with the exterior 

 through the intermediation of the intestine. 



After the appearance of pigment on various parts of the body (velar 



