ECHINODERMATA. 



transverse fibres (p, Jig. 18,) unite the lateral 

 surfaces of the sockets, and serve to approxi- 

 mate them, acting collectively as a sort of 

 sphincter, and as antagonists to those first de- 

 scribed. Lastly, five muscles (figs. 10 and 17, 

 o, o, o,) pass between the semicircular pieces 

 on the upper part of the lantern. Besides the 

 muscles described, there are ten very thin 

 whitish bands (s, s,) which arise in pairs from 

 the external forked extremities of the semi- 

 circular pieces, and are inserted into the border 

 of the shell in the intervals between the arches. 

 Tiedemann describes these bands as muscles ; 

 Meckel, on the other hand, considers them as 

 ligaments ; in the E. esculentus they certainly 

 seem to us to be ligamentous. Two liga- 

 mentous filaments pass from the central end of 

 every semicircular piece to the gullet. A co- 

 vering of the peritoneum envelopes the dental 

 apparatus, extending to it from the border of 

 the shell. 



The oesophagus (fig. 19, a,) rises through 

 the lantern, to which it is connected by fine 

 ligaments, and after a few curvatures termi- 

 nates in a wider part of the alimentary canal, 

 somewhat in the same way as the small intes- 

 tine joins the great in the human body. The 

 wider portion (6, 6,) of the canal turns twice 

 round the inside of the shell in a waving 

 manner, and terminates at the anus (c). In 



. 19. 



Internal view of Echinus sexatilis. 

 A, under half ; B, upper. 



its second or superior circuit it changes to an 

 opposite direction, but its flexures in both cir- 

 cuits are parallel. The tissue of the alimentary 

 canal is very delicate, the external tunic is 

 formed by the peritoneum, which attaches the 

 intestine by a mesentery to the shell, lines the 

 inside of the latter, and is reflected over the 

 ovaries and the lantern. The inner coat of the 

 intestine is soft and of a brownish-yellow co- 

 lour; between it and the external, Tiedemann 

 states that delicate longitudinal and circular 

 muscular fibres are distinguishable. 



The Echini are generally believed to feed on 

 mollusca and Crustacea, and in corroboration 

 of this, Tiedemann states that he has found in 

 the Echinus sexatilis small univalve and bivalve 

 shells entire among the excrements, besides 

 fragments of larger ones. Blainville,* on the 

 other hand, could never find any thing else 

 than sand in the alimentary canal, and he re- 

 marks that the general opinion as to the carni- 



* Diet, des Sc. Nat. art. Owsin. 



vorous habits of the sea-urchin is probably 

 more an inference from the structure of the 

 teeth and jaws than the result of observation ; 

 he, however, adds that M.Bosc had witnessed 

 an echinus in the act of seizing and devouring 

 a small crustaceous animal. In the intestine 

 of the E. esculentus we have usually found 

 numerous small morsels of sea-weed, for the 

 most part encrusted with a flustra. The excre- 

 ments, which are in the form of small round 

 pellets about the size of peppercorns, consist 

 chiefly of sandy matter with fragments of shells, 

 but it would be difficult to say whether these 

 are the remains of digested mollusca or merely 

 a portion of the usual testaceous debris so 

 abundant in sand and mud. 



The principal difference of the alimentary 

 organs in the different genera of Echinida de- 

 pends on the position of the anus and the 

 presence or absence of teeth. In Scutella, 

 Clypeiister, Fibularia, Echinoneus, Galerites, 

 Anunchites, and Spatangus, the anus as well 

 as the mouth opens on the under surface. In 

 Echinus, Cidaris, Cassidula, and Nucleolites, 

 it is situated on the upper surface ; in the first 

 two exactly in the centre, in the last two at a 

 greater or less distance from it. The teeth are 

 wanting in Spatangus and Cassidula. 



c. The alimentary canal of the Holothuria is 



Fig. 20. 



Holothuria tubulosa : alimentary canal and blood- 

 vessels. 

 The respiratory organ, r, c, is cut short. 



