XIV EXPLANATION OF TERMS USED. 



water, when molested, with considerable force. The mouth, or ORAL APERTURE 

 (fig. 5), is usually situated above the anus, or ATRIAL APERTURE (fig. 5J), but in 

 So/ten la the oral aperture is nearest to the stalk, but as the bodj is pendulous it 

 becomes higher than the atrial aperture as usual among the Ascidians. The Tuni- 

 caries exhibit three types of structure, viz., the SOLITARY, the SOCIAL, and the COM- 

 POUND. In the solitary Tunicaries each individual is always distinct ; in the social 

 Tunicaries the organism consists of a number of ZOOIDS permanently connected by 

 means of a vascular canal, or STOLON, through which the blood circulates ; in the 

 compound Tuuicaries the zooids become aggregated into a common mass, but there is 

 no internal union. 



Many of the Tuuicata are found fixed to rocks, either between tide marks or 

 always submerged, while others are oceanic in their habits. 



For further information the student is referred to Woodward's Manual of tlte 

 Mollusca (Weale, 1851), and to Johnston's British Zoophytes (Van Voorst, 1847). 



The classification followed in this Catalogue is the same as that adopted by Dr. 

 Chenu in his Manuel de Conclryliologie (Paris, Victor Masson, 1859), for the shells; 

 and by Mr. Busk, in his Catalogue of the Marine Polyzoa in the British Museum, 

 for the Polyzoa. 



