16 



2. The cuticular system. 



The cuticular system or Perisoma of the Brisinga is composed, as in other star-fishes 

 of the proper integument or skin, and of various calcareous parts which are partly imbedded 

 in the interior of the skin, partly raised above the same in form of spines ridges or micro- 

 scopic pincers, and which exhibit both in structure and function very marked differences. 

 The integument itself also shows a rather different structure in different regions of the body, 

 and we may thus properly distinguish two principal sorts of integuments, the dorsal and 

 the ventral. 



a. The dorsal integument. 



The skin which forms the upper covering of the disc and arms, is of very considerable 

 firmness, almost leather-like, and shews itself (see Tab. I, fig. 8) to be composed of 2 distinct 

 layers or strata, a thicker and firmer interior layer, and a thinner and less substantial ex- 

 terior layer: the former of fibrous, and the latter of cellular structure. In both these 

 strata of the skin there are imbedded various calcareous parts, which will be more particu- 

 larly noticed hereafter. In the middle of the disc and of the arms this dorsal skin is ex- 

 tended quite freely over the subjacent internal organs, and thus forms the immediate boun- 

 dary of the perivisceral cavity in this part; while it joins the ambulacral skeleton on the 

 sides, growing together with the tendinous membrane that envelopes the skeleton like a 

 periosteum. 



The internal stratum of the skin is evidently composed of a contractile tissue; white 

 shining fibres of great tenacity and elasticity being interwoven with each other in different 

 directions; it is especially in the dorsal skin of the disc very strongly developed. No di- 

 stinct pores could be detected in the same, either on the disc or on the arms. 



The external stratum or epidermis is of a much softer and more fragile consistency 

 and partly lined with fine vibratile cilia. It covers as a continuous layer all the uneven- 

 nesses of the body; and all the spines which issue from the surface of the body thereby 

 receive their peculiar sheath-like envelope. On the dorsal side of the arms it forms more- 

 over the peculiar transversal semi-cylindrical fleshy ridges, especially characteristic of this 

 species, which are traceable even to the extreme point of the arm. 



b. The ventral integument. 



From the integuments properly so-called we distinguish the cuticular stratum, which 

 is found on the ventral side inside the ambulacral skeleton, partly lining the bottom of the 



