30 INTRODUCTION TO CLASSIFICATION. 



which are developed in various parts of the walls of the atrial 

 system. 



The precise characters of the true vascular system of the 

 Brachiopoda probably require still further elaboration than they 

 have yet received ; and the same may be said, notwithstanding 

 the valuable contributions of F. Miiller and of Lacaze Duthiers, 

 of their development ; but the shell, the pallial lobes, the in- 

 testine, and the nervous and the atrial systems, afford characters 

 amply sufficient to define the class. 



X. THE ASCIDIOIDA. 



These, like the Brachiopoda, are marine animals, and are 

 very common all over the world ; the more ordinary forms of 

 them being always easily recognisable by the circumstance 

 that their external integument is provided with two prominent, 

 adjacent apertures, so that they look very much like double- 

 necked jars (Fig. 11). At first sight you might hardly suspect 

 the animal nature of one of these singular 

 organisms, when freshly taken from the sea ; 

 kut ^ y ou t uc h i* the stream of water 

 which it squirts out of each aperture reveals 

 the existence of a great contractile power 

 within ; and dissection proves that this power 

 is exerted by an organism of a very con- 

 siderable degree of complexity. Of the two 

 apertures, the one which serves as a mouth 

 is often but not always surrounded by a 

 circlet of tentacles (Fig. 12, c). It invari- 

 ably leads into an exceedingly dilated pha- 

 Fig. 11 1. Phailusia men- t j ie g^es of which are, more or less 



tula; a, oral; b, atrial * 



aperture; c, base of extensively, perforated. The gullet come* 

 off from the end of the pharynx, and then 

 dilates into the stomach, from which an intestine, usually of 

 considerable length, is continued to the anal aperture. The 

 latter is almost always situated within a chamber which opens 

 externally, by that second aperture upon the exterior of the 

 test, to which I referred just now. Furthermore, in all Asci- 



