MUSCULAR SYSTEM. 



537 



as it were the limbs of Crustaceans and other 

 Articulata. 



The study of the muscular system in the 

 extensive class ECHINODEUMATA, the last of 

 the nematoneurose division of the animal world, 

 is invested with considerable interest on account 

 of the very different kinds of locomotive appa- 

 ratus that successively make their appearance ; 

 for as the outward form of these elaborately con- 

 structed creatures changes through all the phases 

 represented by the Encrinite, the Starfishes, 

 the Echinus, the Holothuria, and the worm- 

 like Siponculus,t\\e muscles successively assume 

 a different arrangement. 



The Encrinite in its outward form might be 

 mistaken for a polyp, the jointed calcareous 

 stem whereby it is fixed to the rock, the body 

 and rays around the mouth, as well as the 

 appendages to the stem, being all in their essen- 

 tial structure exactly comparable to those cor- 

 tical polyps, that have an internal jointed cal- 

 careous basis of support. The numerous 

 pieces that compose the skeleton of the Encri- 

 nite are all invested with a living contractile 

 crust, whereby, in fact, they were secreted, 

 and which forms the bond that connects them 

 together. The living crust that covers the 

 Encrinite can scarcely, indeed, as yet be looked 

 upon as being muscular, so soft and acrite 

 does its composition appear to be ; nevertheless 

 in these Echinoderms it seems to be the only 

 moving power employed, and by its slow con- 

 tractions bends the arms, or rays, or stem in 

 any given direction. 



In the long-armed Starfishes, such as the 

 ContutuUe, Gorgonocephali, and Ophiuri, the 

 slender and flexible rays around the body are 

 in like manner covered with a living contracting 

 skin, more dense and coriaceous than that of 

 the Encrinite, but still presenting very dubious 

 appearances of muscular fibre, whereby the 

 movements of the rays are effected. The rays 

 themselves constitute the instruments of pro- 

 gression, and by their aid these Polyp-like 

 creatures crawl at the bottom of the sea, or by 

 entwining them around the sea-weed that covers 

 the rocks climb in search of food. 



In Asteriiis and kindred forms the exterior 

 of the body is still encrusted with the same 

 contractile covering, and can be bent to a 

 certain extent; but in these short-armed Star- 

 fishes the rays have become so short and devoid 

 of flexibility that they can no longer be useful 

 for the purposes of locomotion : an additional 

 muscular apparatus is therefore now conferred 

 in the extraordinary system of protrusible 

 suckers, that become the chief agents in walk- 

 ing, or in seizing prey. 



As we advance from the Asterida we find 

 that the rays at length totally disappear: the 

 body assumes a pentagonal form (Palmipes), 

 then circular (Scutella), and at last is enclosed 

 in an ovoid or globular shell, as in Echinus, 

 Cidttris, &c. In these spherical Echinoderms 

 the external soft and living crust that still 

 covers the exterior of the shell presents obvious 

 claims to muscularity, more especially where 

 it passes on to the articulated spines that are 

 attached to the surface of the shell and now 



become the principal means of progression. 

 The suckers, however, that formed the only 

 locomotive organs in the Asterida still are met 

 with in the Echinida, these and the spines 

 constituting an apparatus for locomotion, which, 

 for its complexity, is unparalleled in the ani- 

 mal creation. In the Echinidae, moreover, a 

 strangely constructed set of dental organs are 

 developed, and for these likewise special muscles 

 are appointed, for a description of which the 

 reader is referred elsewhere. (See Ecu i NO- 

 DERM ATA.) 



In the Holothuridie the shell of the Echinus 

 is no longer secreted, and the living integument 

 itself constitutes the whole parietes of the body, 

 which now becomes quite soft and flexible, 

 clearly commencing that transition which is to 

 connect the Echinodermata with the Annulose 

 division of the animal kingdom. The suckers 

 of the last family are, however, still persistent 

 and form the principal means of moving from 

 place to place. The texture of the fleshy skin 

 of the Holothuria is dense and coriaceous, and 

 strong bands of muscular fibre arranged in five 

 divisions pass in a longitudinal direction, from 

 one end of the body to the other, between 

 which circular and transverse fasciculi are dis- 

 tinctly perceptible. Imbedded in the muscular 

 walls that enclose the visceral sac of the Ho!o- 

 thurida?, delicate nervous filaments are to be de- 

 tected passing along the body from end to end, and 

 most probably these are connected together by 

 a circular filament surrounding the oesophagus. 

 Ganglia, if they exist at all, have, from their 

 minute size, hitherto escaped observation ; and, 

 as might be expected from such a condition of 

 the nervous system, the muscular contractions 

 of the fibrous integument, although associated 

 through the medium of the nerves, and con- 

 sequently far stronger and more energetic than in 

 the lower asteroid Echinoderms, are still almost 

 entirely uncontrolled by the influence of volition ; 

 nay, so remarkably is this the case, that in 

 most species of Holothuria, upon the applica- 

 tion of the slightest stimulus to the exterior of 

 the body, or even by simply taking them out 

 of the water in which they live, such violent 

 and general contractions of the whole integu- 

 ment are excited that the intestines and other 

 viscera are forced extensively through the anal 

 orifice, and it is almost impossible for the 

 anatomist to procure a specimen of these crea- 

 tures without finding it more or less spoiled 

 from this circumstance. 



Lastly, in the Siponculi the vermiform ap- 

 pearance is completely established, the longi- 

 tudinal and circular muscles that bound the 

 visceral cavity are strongly and distinctly deve- 

 loped, the complicated apparatus of foot-like 

 suckers has disappeared, minute ganglia are 

 visible towards the anterior end of the body, 

 and we arrive at the annulose condition, that 

 characterizes the next great division of the 

 animal creation, which now offers itself to our 

 contemplation. 



HOMOGANGLIATA (Owen). The third great 

 natural group of living beings consists of crea- 

 tures having the exterior of their bodies divided 

 into rings or segments arranged behind each 



