MUSCULAR SYSTEM OF THE INVERTEBRATA. 41 



bodies ; the cilia which in these animalcules, serve the double pur- 

 pose of locomotive and respiratory organs, have been observed in 

 every class of animals, even the mammalia. 



In the zoophytes, the soft, fleshy mass, which we have seen 

 secrete the solid matter of the skeleton, possesses distinct, but lan- 

 guid irritability: no part of these animals is, however, so irritable 

 and contractile as their prehensile sacs or polypi. It is not till we 

 arrive at the stellerida and asterias that distinct muscular fibres 

 have been satisfactorily demonstrated ; and in the echinida strong 

 adductor and abductor muscles are seen attached to, and moving the 

 jaws of several species. 



Articulata. — Seeing that these animals possess an extensive sur- 

 face for respiration, and a highly developed nervous system, we are 

 prepared to meet with considerable muscular energy. Accordingly, 

 in the nematoid entozoa, strong muscular fibres are seen taking 

 different directions: the rotiferous or wheel-animals, are also re- 

 markable for the development of the muscular apparatus provided 

 for the movements of their jaws, and long vibratile cilia. The 

 common earth worm has distinct muscles appropriated to the move- 

 ments of its conical pointed feet, and its anus is well furnished with 

 levator and sphincter muscles. The insects and the air-breathing 

 arachnida possess a well marked muscular system ; but of all the 

 branchiated invertebrata, the Crustacea possess the largest proportion 

 of muscle ; this they require for the purpose of swimming, escaping 

 from danger, &c. Although these animals are provided with ex- 

 ternal organs of mastication, the interior of their stomach is set 

 with numerous teeth, and its exterior provided with a suitable 

 arrangement of muscle. 



Mollusca. — The muscular system presents much greater variety 

 of form in this than in the articulate class. The tunicata possess 

 a distinct muscular coat within their cartilaginous covering, by 

 which they act on their entire body, and empty their thoracic 

 cavity; the orifice of which, as well as the anus, is provided with 

 sphinctorial muscles. The foot of the conchifera, by which they 

 swim, creep, burrow, or attach themselves, is hollow, and composed 

 almost entirely of muscular fibres, taking different directions. This 

 foot occasionally admits the water into its interior, and is absent 

 from the oyster and others where the shell is permanently fixed. 

 In the gasteropoda, the foot is the largest muscle in the body, and 

 in trachelipodous gasteropods or those residing in turbinated shells, 

 its fibres are traceable up to the neck, and backwards to be attached 

 to the shell. The predaceous gasteropods possess a powerful mus- 

 cular proboscis provided with a fleshy tongue, armed with sharp 

 conical teeth, as seen in the common whelk, (buccinum undatum.) 

 In the chephalopoda and pteropoda, muscular fins are attached to 

 the side of the trunk for progressive motion. And in the naked 

 cephalopods, a thin panniculus carnosus with interlacing fibres, is 

 spread all over the body, beneath the coloured skin of the mantle. 

 The octopus is destitute of any lateral fins, but is provided with a 



