REPORT ON THE HOLOTHTJRIOLDEA. 121 



of canals. Besides these transverse appendages, one or several pairs of more or less 

 rudimentary processes seem to be present as a rule. 



The Body-wall. 



The structure of the solid sac termed the body-wall or the perisoma, which encloses 

 the spacious peritoneal cavity and gives to the animals their form, has been already 

 very satisfactorily described by Baur, 1 Semper, 2 Teuscher, 3 &c, and, the material being 

 so softened and macerated by a long immersion in spirit as to make a closer histological 

 examination impossible, I have nothing of importance to add to their investigations, and 

 refer to the splendid memoirs of these authors. Notwithstanding the previously-known 

 facts, I think it may be well to state some points which I have observed when examin- 

 ing the deep-water forms, and which may possibly aid to throw light upon the whole 

 class. 



The body-wall consists, as already known, of an outer cellular ectoderm, covered 

 externally with a very thin, transparent cuticule ; beneath the ectoderm is a layer of 

 connective tissue, the corium, within which are to be seen circular and longitudinal 

 muscular fibres lined internally with a delicate peritoneum. Corium is of the greatest 

 importance not only in being the secreting layer of the calcareous deposits peculiar to the 

 Echinoderms, but also by reason of its more or less considerable thickness, which occa- 

 sions striking changes in the shape of the body. The thickness of this layer of con- 

 nective tissue varies much in different species, and even in the same individual is not 

 always uniform throughout the whole body. According to Selenka, 4 the body-wall in 

 the genus Stiehopus, Brandt, is always thicker along the interval between the ventral 

 and dorsal surfaces; the same applies to the Elasipoda, in which Deima and Oneiro'plianta 

 form good examples, having the large, branched, ambulacral cavities of their lateral 

 pedicels and processes lying inside the thick layer of connective tissue. This peculiarity 

 distinguishes to a very great extent the whole of the Psychropotidas, the representatives 

 of which have the perisoma increased in thickness all around the body, so as to form a 

 more or less considerable brim (PI. XL. fig. 6). The singular large appendage which is 

 present on the back in a great number of Elasipoda is likewise for the most part made up 

 of connective tissue. 



The tentacles, the pedicels, and the processes are composed of the same layer of 

 tissues as the body- wall proper, excepting that no circular muscular fibres are to be found. 

 In various forms, as, for instance, in Deima validum, &c, the layer of connective tissue, 



1 Beitriige zur Naturgeschichte cler Synapta digitata, 3 Abhandlungen, mit 8 Tafeln (Nov. Act. Acad Cces. Leop. — 

 Carol., torn, xxxi., Dresden, 1864). 



2 Reisen im Archipel der Philippinen, ii., 1, Holothurien, Leipzig, 1868. 



3 Beitriige zur Anatomie der EcMnodermen (Jenaische Zeitschr. £ Naturwiss. Bd. x., Jena, 1876). 



4 Beitriige zur Anatomie und Systematik der Holothurien (Zeitschr. fur wissensch. Zoologie, xvii., 186", p. 315). 



(ZOOL. CHAL. EXP. — PAET XIII. — 1881.) N 16 



