62 



EXAMPLE OF CONNECTION BETWEEN SUB-KINGDOMS. 



so exactly repeated, that it is not easy to distinguish the group to 

 which certain species belong, by a cursory glance at their exterior. 

 42. Lastly, we may advert tj an instance, in which even 

 two sub-kingdoms are connected, by links of transition so close, 

 that it is not easy to say where one begins and the other termi- 

 nates. Among the Mollusca, but occupying the lowest place in 

 that division, are usually ranged a collection of animals, which 

 do not possess a shell, but which have their organs enclosed as it 

 were in a bag or tunic ; from which they derive the name of 

 Tunicata. None of them have any considerable power of spon- 

 taneous movement ; and a large proportion of them are fixed 

 during their whole lives. Many of them are social in their 

 habits ; that is, a number of them live together, adhering to each 

 other by their external surfaces. But there are some which not 

 only adhere together, but which are more closely connected ; 

 a number being included in one common envelope ; or being 

 attached singly to a connecting stem, as shown in Fig. 23. 



Now, in these last cases, 

 they propagate, not only 

 in the manner of the other 

 Mollusca, by the deposi- 

 tion of eggs, but also in 

 the manner of the Po- 

 lypes, by gemmation or 

 budding ; for all the ani- 

 mals forming one cluster, 

 or mounted upon one 

 stem, proceed from a 

 single individual, which has thus extended itself in the manner 

 of a Plant. The simplicity of the structure of these animals is 

 very great. The tunic has two orifices, b and , through the 

 former of which, water is continually drawn in, whilst through 

 the latter it is as constantly expelled. The liquid enters a large 

 chamber, which may be regarded as a dilated pharynx or gullet ; 

 and at the bottom of this lies the entrance to the stomach, e, 

 from which the intestinal tube, i, passes to the orifice of dis- 



FIG. 23. SOCIAL ASCIDIJE. 



