474 THE ANATOMY OF INVERTEBRATED ANIMALS. 



endoderm, becomes the muscular and peritoneal coat of that 

 viscus ; while the outer wall, attaching itself to the ectoderm, 

 or to the mesoblastic cells which line it, is, with them, con- 

 verted into the muscular and peritoneal investment of the 

 parietes of the bod v. The interspace between the two is the 

 peritoneal cavity. 



In the mean while, the body of the embryo elongates, the 

 tentacula are developed around the mouth, the ciliated bands 

 disappear, and the Holothurid Echinoderm is complete. 



Thus it is clear that the peritoneal cavity of the Holo- 

 thurid is an enteroccele, and that it answers to the perivis- 

 ceral cavity of Sagitta, or of the Brachiopoda ; and further, 

 that the ambulacral vessels are also modifications of the entero- 

 ccele. Moreover, it is obvious that the structures which are 

 developed between the enteroccele and the ectoderm and en- 

 doderm, answer to those which are evolved from the meso- 

 blast in other animals ; and that the adult Echinoderm stands 

 in the same relation to the Echinopwdium as an Annelid does 

 to its embryo ; the adult form being due to the peculiar ar- 

 rangement of the parts developed from the mesoblast. No 

 part of the Echinopaedium is cast off in the course of the de- 

 velopment of the Holothuridea. 



2. The Asteridea. — A Star-fish is comparable to a Holo- 

 thurid, the ambulacra of which are restricted to its oral half, 

 flattened out so as to have a very short axis ; while its equa- 

 torial diameter is greatly increased, and produced in direc- 

 tions corresponding with each ambulacrum. The result would 

 be a disk, having the form of a pentagon, or of a five-rayed 

 star, with ambulacra only on that face of the disk which 

 bears the mouth. Hence the ambulacral, and the opposite, 

 or antambulacral, faces are of equal extent. 



Most Asteridea are like five-rayed stars, but some are 

 pentagonal disks (Goniaster), and some few (Solaster) have 

 more than five rays. In Brisinga, the rays are much more 

 different from the disk than usual, and the genus thence 

 acquires an outward resemblance to an Ophiurid. 



All the Asteridea are provided with a skeleton made up 

 of plates or thick rods, composed of a dense calcareous net- 

 work. A deep groove, radiating from the mouth to the end 

 of the ray, marks the position of each ambulacrum, and the 

 sides of this groove are supported by two series of ambu- 

 lacral ossicles, which meet and articulate together in the 

 middle line or roof of the groove. The ambulacral nerve and 



