504 SUMMARY OF CURRENT RESEARCHES RELATING TO 



5. Lamellibrancliiata. 



lamellibranchiata without gills.* — M. P. Pclscneer has been able 

 to confirm the rcniarkablo observation of Mr. Dall that Cuspidaria has no 

 gills. On raising the mantle one finds oneself in the 2)rcseuco of a mus- 

 cular surface \\hich Dall regarded as the body-wall. Tliis surface is a 

 partition which separates a dorsal from a ventral chamber ; it is traversed 

 by the foot, and extends from one adductor to the other ; on either side 

 it is connected with the mantle, which is continuous along its whole 

 length ; posteriorly it is connected with the partition which separates 

 the two siphons. The visceral mass is found in the dorsal chamber. 

 The labial palps are present, but are very small. 



The study of the allied genera LyonsleUa, Poromya, and Silenia has 

 resulted in the unexpected discovery that the muscular septum is a 

 modified gill. In Lyonsiclla abyssicola the gills are united to the mantle, 

 fused with one another behind the foot, and then joined to the division 

 between the two siphons ; but the structure of the gills is preserved. 

 In Poromya there is a similar partition, but this is muscular ; on either 

 side, however, there are two groups of branchial lamellfe, separated from 

 one another by clefts which allow of a communication between the two 

 pallial chambers. In Silenia the reduction is still greater, for the 

 branchial lamellae have disappeared, and the clefts have become arranged 

 in three separate groups. In Cuspidaria reduction is brought to an 

 extreme. 51. Pelseneer proposes to form a separate group for the last 

 three genera, and to call it the Septibranchia ; Cuspidaria must form the 

 type of Ball's family Cuspidariidte. 



So-called Eyes of Tridacna and Occurrence of Pseudochlorophyll 

 Corpuscles in the Vascular System of Lamellibranchs.t — Herr J. Brock 

 gives an account of the so-called eyes which aid so largely in giving a 

 eplendid coloration to the margins of the mantle of living species of 

 Tridacna. They form an irregular row of difierently coloured points, 

 and look like gems. The method employed by Vaillant did not permit 

 him to successfully investigate the minute structure of these organs. 



The larger wart-like elevations which are found at some distance 

 from the margin of the mantle agree in structure with the mantle itself. 

 In the warts, however, there are a few peculiarly constructed minute 

 organs which might be taken for eyes. These bodies are flask-shaped, 

 and have their long axis perpendicular to the surface of the epithelium ; 

 the whole organ is surrounded by a thin membrane in which fusiform 

 nuclei are scattered. Within are large cells, also with a distinct mem- 

 brane, and containing clear, and j)robably highly refractive protoplasm. 

 These transparent cells are surrounded bj'^ a layer which is characterized 

 by its great irregularity, and the component cells of which contain 

 coarsely granular protoplasm. No nerve was in any case seen to pass 

 to a flask-shaped organ. 



The author is unable to make any suggestion as to the function of 

 these organs, but he thinks it may be confidently asserted that they 

 are not optic. It is much more probable that they are luminous organs; 

 if the cells of the outer layer have the faculty of shining, the more trans- 

 parent inner cells may act as prisms. The only bodies which can be 



* Comptes Eendus, cvi. (1888) pp. 1029-31. 



t Zeitschr. f. Wisa. Zool., xlvi. (1888) pp. 270-88 (1 pi.). Transl. Ann. and 

 Mag. Nat. Hist, i. (1888) pp. 435-52. 



