12 COMPARATIVE ANATOMY. 



circle of oral tentacles, above the calcareous ring. The ventral 

 surface is covered with calcareous scales, and is destitute of 

 tube-feet : though these latter do exist in other parts of the 

 body. 



20. A specimen of Holothuria tubulosa, dissected to shew 

 the generative system and * respiratory tree/ The greater 

 portion of the left side of the body has been removed, together 

 with the whole of the digestive tract, except the pharynx and 

 cloaca. These are well displayed, as also are the mesenteric 

 bands by which the cloaca is attached to the body-walls. 



The generative caeca have been spread out on a piece of 

 mica. They are disposed in two groups, but are shorter and 

 much less subdivided than in P solus, and end in dilated casca. 

 The two ducts, one from each group, shortly unite to form the 

 common efferent duct, appended to which, at about the level of 

 the calcareous ring, is a number of minute pyriform vesicles. 

 The external opening of the generative duct has a black bristle 

 inserted into it. The respiratory tree consists of a hollow stem 

 extending from the cloaca, with which it is in communication, 

 as far forwards as the pharynx. Numerous filiform and arbo- 

 rescent caBcal appendages are given off from it. The mesen- 

 teric fibres by which the stem is attached to the body-wall are 

 well seen. 



On the left of the preparation is the 'respiratory tree,' 

 attached by mesentery to the body-wall. A green rod has 

 been inserted into the orifice by which it opens into the 

 'cloaca/ Depending from the calcareous ring are the 'ampulla' 

 of the water- vascular ring surrounding the mouth. 



SUB-KINGDOM VERMES. 

 DIVISION ANNULATA. 



Class I. ANNULATA PROPER. 



21. An Amphinome. The body, though elongated, is of 

 considerable transverse width, and is divided by numerous 

 transverse constrictions into about 60 segments. With tlie 



