28 COMMISSION OF CONSERVATION 



view, Brooks did not mistake the blastopore, but mistook the place of origin of the 

 shell (and, implicitly, of the shell-gland, although this was unknown to him.) 



In the orientation of the larva with respect to the constant direction of gravitation, 

 i.e. the fixation of dorsal and ventral, anterior and posterior, right and left, the organs 

 of earliest and easiest recognition, such as blastopore and prototroch, shell, mouth, 

 and anus, are of first importance, and it is of great advantage to preserve a constant, 

 continuous, and uniform method, such as renders it unnecessary at some particular 

 stage to turn the larva upside down for comparison with the egg on the one hand or the 

 adult oyster on the other. This is what Brooks did at the stage of his fig. 37 (1880 but 

 not 1905), and what Nelson did at fig. 7 (1902). It requires some rearrangement of 

 this part of their works, in the light of Horst's observations (1884), to make a logical 

 presentation. 



With the first appearance of the prototroch it becomes clear which is the anterior 

 end. With the first appearance of the shell it is plain what is the dorsal surface. It is 

 then evident that the blastopore (as well as the permanent mouth) is ventral, and that 

 it is possible to return and, in a general way, to fix directions from the very first cleavage. 

 Even before this, dorsal and ventral may be marked out by the position of the polar 

 bodies and active protoplasm above, and of the deutoplasm below, but for the rest there 

 is radial sjTnmetry about the chief (vertical) axis. The first fission is transversely 

 vertical, resulting in an incomplete anterior blastomere (united with the deutomere to 

 form a macromere) and a more complete posterior blastomere (micromere). The 

 symmetry is radial with two rays, giving it the appearance of being bilateral — the two 

 rays being necessitated by the process of division. The second fission is longitudinally 

 vertical, but it is scarcely justifiable to claim that two blast omeres are anterior and two 

 posterior. The deutomere adjusts itself more particularly to one of these, which, if 

 bilateral symmetry predominates, must be the anterior one, leaving the other three 

 respectively posterior, right and left. This shifting of the plane of bilateral sjonmetry 

 shows that the latter is not yet estabhshed. Radial symmetry (here with four rays) 

 exists, and continues (with increasing number of rays) to exist throughout the process 

 of segmentation, up to the completion of the gastrula. During this time the area of 

 most activity changes from dorsal to posterior, then anterior, and finally ventral; and 

 there are alternations of apparent bilateral with radial symmetry. Radial sjTnmetryis 

 characteristic of the egg, oosperm, and embryo, during their floating and resting con- 

 ditions; bilateral symmetry first becomes prominent with the free-swimming larva. It 

 would seem that gravitation first determines dorsi-ventrahty, and with it horizontal 

 radial symmetry, which becomes more emphatic with growth along compass lines of 

 equal pressure, and tends to be sustained by habits of rest, flotation, fixation; and 

 that automatic locomotion through a resisting medium necessitates especially a mon- 

 axial gi-owth of the body, a continual precedence of one end, and an equal balancing of 

 right and left sides, i.e. bilateral sjrmmetry. 



In the origin of the permanent mouth from (or in the position of) the blastopore. 

 Teredo, Cardium, Modiolaria, and other genera of Lamellibranchia agree -ndth or ap- 

 proach to Ostrea. This appears also to be the rule among marine Gastropoda (e. g. 

 Patefla), Scaphopoda (Dentalium), and Amphineura (Chiton). But according to 

 various earlier observations, there are exceptions or modifications from this, chiefly 

 among fresh-water Unionidse and Pulmonata, where the blastopore may become the 

 anus (Pisidium, Paludina) , or extend as a slit from mouth to anus (Lymnseus, Planorbis) 

 or become closed on the dorsal surface and have no relation to either mouth or anus 

 (Anodon). In the fate of the blastopore, as in segmentation, there appears to be so 

 little unity among genera that are classed together as to make renewed, special, and 

 extensive observations desirable. 



The Shell-bearing Larva.— It has been already pointed out that the 

 beginning of swimming locomotion marks a convenient halting-place for 

 stock-taking in organization, and that with regard to the latter the proto- 

 troch is the most conspicuous feature. The next most recognizable char- 

 acter to serve as a mile-post in the train of events is the shell. The form- 

 ation of a shell-gland, and of a shell, furnishes the first distinctive indi- 

 cation of the molluscan origin of the larva, and separates the period of the 

 pre-conchiferous trochophore from that of the conchiferous veliger. The 

 trochophore, or a slight modification of it, is the larval form of several 



