704 DE. W. B. CAEPENTER ON THE STEUCTUBE, PHYSIOLOGY, AND 



freedom of movement between them ; whilst others are decidedly elastic, their action 

 being to antagonize muscles, as in many other well-known cases among Vertebrate and 

 Invertebrate animals. Where a firm union is required, without power of movement 

 between one segment and another, — as we shall find to be the case in regard to the 

 pieces which form the basis of the calyx, — there is no ligamentous connexion, but simply 

 an adhesion of expanded surfaces, closely fitted to each other, and held together by the 

 continuity of their sarcodic basis-substance. 



19. The segments of the Arras and of their lateral pinnae, and to a certain extent 

 those of the Rays which bear them, are made to move one upon another by a highly- 

 developed Mtiscular apparatus. This consists of fibres (Plate XX,III. fig. 4) which 

 resemble those of the ordinary muscles of Terehratulce^ in their general appearance 

 and their want of mutual cohesion. They are cylindrical, or somewhat flattened, and 

 show no trace whatever of transverse striation (Plate XLIII. fig. 4, a). Their diameter 

 is about Ta-Jijx) of an inch ; and I have not been able to resolve them into more minute 

 elements. Interspersed among them ai-e numerous spheroidal corpuscles ranging in 

 diameter from about m^ao to ^q^q of an inch : similar corpuscles of hemispherical or 

 elongated form are frequently to be seen adhering to the edges of the fibres by their 

 flattened faces (a, b) ; and sometimes elongated corpuscles are obsenable, over which the 

 border of the fibre seems continuous {c). What is the Histological import of these cor- 

 puscles, does not seem very clear. I do not find that either they or the fibres are spe- 

 cially affected by the ordinary reagents. The fibres expand a little at their terminations 

 (fig. 4, a), so as to come into closer union than elsewhere ; and these expanded terminations 

 are simply applied to the surfaces of the calcareous segments to which they are attached, 

 not passing into their substance ; so that the muscular bundles are easily torn away en 

 masse, leaving no such roughness behind as when a ligamentoiis connexion is similarly 

 treated. From the entire absence of anything like Sarcolemma or Connective tissue, the 

 fibres are very easily isolated from each other ; and there is no difficulty in tracing the same 

 individual fibres from one point of attachment to the other through the whole length of 

 the muscular bundle, which is sometimes as much as -^ of an inch. The entire absence 

 of any other component in the substance of these Muscles is a point of no little interest 

 After careful and repeated examination of them, I feel justified in stating that they show 

 no trace either of Blood-vessels or Nerves; yet the evidence already given (§§ 9, 13) 

 from observation of the habits of the living Antedon, shows that in energy and rapidity 

 of muscular action it surpasses every other known animal of its class. When we come 

 to study the Nutritive apparatus, we shall find that although no Blood-vessels pass into 

 the substance of the Muscular bundles, their surface forms the floor of a canal filled 

 with nutritive fiuid, for the aeration of which there is a special provision in the wide 

 expansion of the Arms. The mode in which Nervous influence is conveyed to them is 

 a problem of greater difficulty. It will be shown in the Second Part of this Memoir, 



' See Mr. Hancock's Memoir " On tte Anatomy of the Brachiopoda," in Philosophical Transactions for 1858, 

 p. 804. 



