178 ECHINODERMATA. 



no diminution in the size of the main trunks as they approach 

 their termination ; and the cross branches given off in their course, 

 instead of ramifying, all end in the minute ambulacral vesicles, to 

 the injection of which they would appear to be subservient. 



(218.) The generative system of the Holothuria is essentially 

 similar to that found in the Asteridse, consisting of long ovigerous 

 caeca, without any superadded parts which might be regarded as con- 

 tributing to the impregnation of the ova. The germs are secreted 

 in slender ramified tubes (Jig. 74, A, h,) which are collected into 

 one great bundle, and open externally by a common canal in the 

 neighbourhood of the mouth, not into the oesophagus as Cuvier 

 supposed, but upon the back of the animal. These generative 

 cseca at certain times of the year become enormously distended, 

 being at least thirty times larger than when not in a gravid state ; 

 if examined at this period, they are found to contain a whitish, 

 yellowish, or reddish fluid, in which the ova are suspended, but 

 nothing is known concerning the mode of the expulsion of the 

 eggs, or their subsequent developement. 



(219.) 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 

 in which they are imbedded, it is next to impossible to display their 

 course ; most probably, as in the Echinus and Asterias, these com- 

 municate with a circular cord which embraces the oesophagus. No 

 ganglia have as yet been discovered even in the Holothuria ; and 

 consequently, although the muscular actions of the body are no 

 doubt associated by nervous filaments, the movements of these 

 creatures appear due rather 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 irritation 

 applied to the surface of the body causes such powerful contrac- 

 tions of the integument 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 circum- 

 stance as to have induced the older anatomists to suppose that, by 

 a natural instinct, the animals when seized vomited their own 



