MUSCULAE SYSTEM OF ECHINODEEMA. 207 



and Holot]iurite. They sometimes form larger firm pieces, such as 

 the scutiform structures, which cover the dorsal or antambulacral 

 surface in Cuvieria, and which though much smaller are widely 

 distributed in the dermis of Echinocucumis. 



In the Holothurire the leathery layer of connective tissue becomes 

 very strong* indeed, but it is quite soft in the Synaptte. In these 

 latter, however, there are calcareous bodies deposited, which are 

 frequently definite in form ; such as the calcareous rotul^e of the 

 Ohirodotge, or the fenestrated plates (Fig. 105, B) into which the 

 bases of the anchor-shaped hooks (A) are inserted. These latter 

 project from the integument, and give to the skin of the Synaptje 

 its great power of attachment. 



The Holothuroida also possess an internal skeleton proceeding 

 from their dermal one. It consists of a calcareous ring surrounding 

 the gullet, which serves as a point of insertion for the muscles of the 

 body, and as a support for the other organs. In the Holothurias it 

 consists of 10, and in the Synaptee of 12-15 separate pieces. In 

 the former five larger pieces alternate with as many smaller ones, 

 and are more or less movably connected with one another. They 

 are homologous with the processes, Avhich project inwards from the 

 oral edge of the shell in the Echinoi'da. Like those processes, 

 they are provided in the Synaptaa with pores, which give passage to 

 the nerves and ambulacra! canals ; in the Holothurise these organs 

 pass out through forked processes. 



Muscular System. 

 § 168. 



In the Echinoderma, as in the Vermes, the muscular system is 

 connected with the integument, and the structures derived from it. 

 The arrangement of it is essentially dependent on the development 

 of the dermal skeleton, so that it is only developed into a system of 

 body muscles in those cases, where the body is able, thanks either 

 to the articulations of its separate firm parts (Astero'ida and 

 Crino'ida), or to the presence of disconnected calcareous deposits in 

 its integument (Holothuroida), to change its form. 



In the Astero'ida and Crinoida the musculature of the arms, like 

 the arms themselves, is jointed; it fills up the interspaces between 

 the solid parts. In the Crinoida, where the skeletal parts of the 

 arms are connected together by elastic tissue, these muscles lie on 

 the ambulacral or ventral surface of the body, and serve principally 

 to bend them, while the elastic tissue between the joints straightens 

 them again. The same arrangement obtains in the pinnulas of the 

 Crinoida. 



This system of muscles is rudimentary in the Echinoi'da, where 

 the perisome is hardened into a firm ^'shelP^ consisting of immovably 



