632 SUMMARY OF CURRENT RESEARCHES RELATING TO 



In the young the disk consists of one central and nine large 

 contiguous triangular pieces, some of which are interbrachial, and all 

 of which carry large mobile spines, together with a few smaller plates, 

 which alternate with these latter and are intermediate between them and 

 the central piece, but considerable changes occur during growth ; the 

 central pieces become separated, the interbrachials are pushed to the 

 edge of the disk, and, becoming placed exactly in the angle of the arms, 

 they end by forming the odontophores. The madreporic plate is 

 always formed on one of the plates of the first row of the disk ; in 

 Brisinga the forcing out of these plates stops when they reach the 

 outer edge of the buccal ring ; if the process of growth bad only con- 

 tinued and carried all these pieces on to the ventral surface, we should 

 have had an Asterid with the ventral surface of an Ophiurid. Here 

 then we see a considerable " rapprochement " between Ophiurids and 

 Asterids, while the earlier arrangement of the plates of the disk 

 seems to M. Perrier to recall the constitution of a Crinoid. 



Anatomy of Holothurians.* — E. Jourdan describes the testicular 

 tube as being formed of three layers ; an external cellular investment, 

 a fibro-muscular zone, and a layer of internal epithelium. In 

 Holothuria tubulosa the majority of the cells of the first of these are 

 large and flattened ; some, however, are distinguished by consisting 

 of a mass of refractive corpuscles within a delicate membrane ; these 

 are feebly coloured grey by osmic acid, and are strongly coloured by 

 methyl-green ; the latter fact would lead one to suppose that they 

 were young cells, while in their general appearance they are to be 

 compared to fat-cells. In Cucumaria and Phyllophorus the cells of 

 this layer can in no way be compared to ordinary epithelial cells ; 

 and it would seem to follow that in some genera of Holothurians the 

 peritoneal elements have acquired a special importance, inasmuch as 

 the normal epithelial cells have completely disappeared. In the 

 median layer we find a connective membrane overlaid by a layer of 

 very fine circular muscles, identical with those which are to be found 

 in the Polian vesicle. 



The study of the internal epithelial layer ought not to be separated 

 from that of the spermatozoa ; the sperm, if examined in spring or 

 summer, is found to consist of a mass of large cells which may be 

 regarded as spermatoblasts ; the cells of these bodies are similar to 

 those which line the walls of the testicular tubes, and are found, when 

 isolated, to be identical with them. Cells, likewise spherical, are 

 also to be made out in which the protoplasm appears to be condensed 

 into a very large nucleus, while first one, and then several, homogeneous 

 refractive corpuscles appear in the spermatoblasts ; the granular 

 protoplasm at last disaj^pears altogether, and in the place of the 

 original spermatoblast we have a cell containing a number of cor- 

 puscles, each of which represents a spermatozooid ; these have, when 

 developed, a spherical head and a very long tail. 



It has been found that each tube of the so-called Cuvierian organ 

 consists of a muscular sheath formed of bundles of longitudinal and a 



* Comptes Bendus, xcv. (1882) pp. 252-4. 



