230 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Shell of Lamellibranchiata.* — An investigation of the structiiro 

 and tlcvclopnient of the shells of a great number of Lanielli'orancliiata 

 bad led W. Miillcr to distinguish two chief varieties — (1) Shells 

 which are only here and there connected with the mantle ; tho 

 organic substance of tho mother of- pearl layer is membranous. 

 (2) Shells which arc continuously grown to the mantle ; the organic 

 substance of the motlier-of-pearl layer forms a network. Only Cijclas 

 belongs to the second group ; tho former contains all other Lamelli- 

 branchs. 



Molluscoida. 

 a. Tunicata. 



Evolution of the Blood-vessels of the Test in Tunicata. f — W. A. 

 Herdman describes the arrangement of the circulatoi-y system in 

 the test of Ascidiaus. The extent to which this is developed varies 

 greatly in the different species. A series of simple Ascidiaus could 

 be formed showing all conditions between the two extremes, and also 

 exhibiting very varied arrangements. The most interesting modifica- 

 tions are in tho genus Culcolus, in which there is a great development 

 of the vessels just on the surface of tho test, and sej^arated from the 

 surrounding medium by a very thin layer of tissue, and it may in this 

 case act as an accessory organ of respiration, an idea sujiported by 

 the condition of the corresponding system of vessels in some of the 

 compound Ascidiaus. 



The first stages in the evolution of the " respiratory " vessels the 

 author imagines to bo as follows : — As the ancestors of the AscidiidsB 

 lost the power of reproducing by gemmation, the vascular stolons 

 became rudimentary, until they were useful merely as adhering organs. 

 For some time they would only be produced at the posterior end of 

 the test (their original position in the Clavelinida)), but in course of 

 time they would extend further forwards along the left side of the 

 body (the side upon which most simple Ascidiaus lie), so as to anchor 

 the animal more securely. They would then probably (in some not 

 very remote ancestor of Ciona) begin, while still acting as adhering 

 organs, to be of some slight use in respiration, and would conse- 

 quently, by the action of natural selection, be evolved gradually into 

 a larger system of vessels. The only difficulty is the passage from 

 the Claveliuid to the Cionid arrangement, fz'om the gemmiparous 

 stolon to the first traces of a respiratory system. This can be ex- 

 plained by assuming that the rudimentary stolons after they had lost 

 their primary function became useful as adlicring organs, and conse- 

 quently were retained or possibly increased by the action of natural 

 selection until their respiratory function became established. 



Microscopic Elements serving for the Determination of the 

 Cynthiidse.:}: — H. de Lacaze-Duthiers describes a method of readily 

 determining the species of Cynthiida) by microscopic characters, 

 ■\vithout, however, injuring the specimens. 



* Zool. Auzcig., viii. (1885) pp. 70-5. 

 t Nature, xxxi. (1885) pp. 247-9 (5 lig.-j.). 

 \ Comptes Keudus, xcix. (1884) pp. 1103-G. 



