44 THE ZOOLOGIST. 



found to be broken in preserved or dissected specimens. When 

 irritated or alarmed a Sea-slug ejects its intestines, but by no 

 means dies on account of this apparently suicidal act ; just like 

 all other Echinoderms it possesses a power of repair amply 

 sufficient to make up for even the most remarkable acts of 

 self-mutilation. 



In Cucumarin, and in a large number of Holothurians, there 

 are connected with the cloaca, into which the intestine opens, a 

 pair of elaborately and exquisitely branched delicate tubes which 

 extend far forwards into the body ; into these water passes from 

 the exterior and is again driven out ; as this supply of fresh 

 water brings with it oxygen, and as this oxygen is only separated 

 by a thin wall from the carbonic acid gas in the body cavity an 

 exchange of gases is effected — the water giving up its fresh 

 oxj^gen, and taking away the carbonic acid or waste product;* 

 owing to this respiratory function the branched tubes are called 

 "lungs." 



There is only one other organ of the Sea-slug to which refer- 

 ence need be made here, and it also is connected with the walls 

 of the cloaca ; it consists of a system of blind tubes which may 

 be very small or very large in number, and may form a compact 

 mass, be branched, or whorled in arrangement. These tubes, 

 which we know by the indifferent name of Cuvierian organs, have 

 had very various functions ascribed to them at different times ; 

 it is now certain that, as shall be shown later on, Prof. Samper's 

 supposition that they were organs of offence is correct. 



Slight as our knowledge of Holothurian organisation is from 

 this sketch, it is sufficient to enable us to understand the 

 principles on which the group is classified; to the consideration 

 of its arrangement we will, therefore, now proceed, taking on 

 our way opportunities of saying a few words about the more 

 interesting examples which are to be found in our own seas. 



If we start with such a typical form as the Cncumaria which 

 has formed the basis of the preceding account, we find we have 

 had to do with an elongated animal whose body is divided by 

 five rays into five parts, that the suckers are confined to the 

 rays, that there are five pairs of tentacles, and scattered spicules 



-■■ Compare " Some of the Ways in which Animals Breathe," Zool. 1886, 

 pp. 3Uo— 318. 



