FISTULAEID^. 209 



strength, and the little creature is transformed into the shape and at- 

 tains the proportions of its parent. 



(541.) The special instruments of touch, the only sense allotted to 

 these animals, are the branched tentacula around the mouth, which 

 seem by far the most irritable parts of the body. The nervous system 

 is so obscurely developed, that even Delle Chiaje was unable to detect 

 any traces of its existence ; nevertheless there is little doubt of the 

 presence of nervous threads in the muscular envelope of the animal, 

 although, from the dense tissues wherein they are imbedded, it is next 

 to impossible to display their 6ourse ; most probably, as in the Echinus 

 and Asterias, these communicate with a circular cord that embraces 

 the oesophagus. No ganglia have as yet been discovered even in the 

 Solothuridce ; and consequently, although the muscular actions of the 

 body are no doubt associated by nervous filaments, the movements of 

 these creatures appear rather to be due to the inherent irritability of the 

 muscular tissues themselves, than to be under the guidance and control 

 of the animal. In many species, the slightest mechanical irritation 

 causes such powerful and uncontrollable contractions of the integu- 

 ment, that the thin membranes of the cloaca, unable to withstand the 

 pressure, become lacerated, and large portions of the intestine and other 

 viscera are forced from the anal aperture. So common, indeed, is the 

 occurrence of this circumstance, that the older anatomists were induced 

 to suppose that, by a natural instinct, the animals, when seized, vomited 

 their own bowels. It is, in fact, extremely difficult to obtain perfect 

 specimens of the Holothurida3, from the constant occurrence of this 

 accident : but, although annoying to the naturalist, such a phenomenon 

 affords the physiologist an important lesson, teaching that here, as in 

 the lower Zoophytes, the muscular system possesses an innate contractile 

 power, which would seem only to be destroyed by incipient putrefaction ; 

 but so little is this contractility under command, that, once excited to an 

 inordinate extent, it becomes totally unmanageable, even though its 

 continuance inevitably causes the evisceration of the creature. 



(542.) FISTTJLAKIDJE. In order to complete our account of the organi- 

 zation of the Echinodermata, we have still to investigate the structure 

 of the Fistularidce a group that, from the external appearance of the 

 individuals composing it, and the total absence of the tubular feet met 

 with in other families, has been improperly separated by some modern 

 writers from the class under consideration. Nevertheless we shall find 

 the position assigned to these animals by Cuvier to be in strict accord- 

 ance with the character both of their outward form and internal struc- 

 ture ; only, instead of placing them with the lowest of the Echinoderms, 

 they would have been more properly situated at the head of the class, 

 as most nearly approximating the Annelida in all the details of their 

 economy. We have already given a description of the outward form of 

 a Fistularia ( 443), and seen the completely annulose condition of its 



p 



