Z BRITISH TUNICATA. 



We shall, therefore, consider the Tunicata as the 

 lowest members of the Molluscan series, touching, on 

 the one hand, the Lamellibranchiata, on the other the 

 Bryozoa and the Brachiopoda. They are all marine, 

 shell-less, headless, footless mollnsks of a comparatively 

 low organisation, having an elastic external envelope, 

 test, or outer tunic, the homologue of the ordinary 

 molluscan shell, and an internal tunic or mantle ; they 

 are provided with a well-differentiated digestive appa- 

 ratus, usually with a sac-shaped gill placed in front 

 of the digestive organs, and leading to the mouth ; 

 with rather well-developed muscular organs, in which 

 the heart is tubular, forcing the blood for a while in 

 one direction, and then reversing its action and 

 forcing it in the opposite direction. They are all 

 androgynous, with very complete reproductive organs, 

 and all undergo a metamorphosis, while the greater 

 number of them increase by buds as well as by ova. 



They are mostly attached, apathetic creatures, en- 

 joying locomotion only in their first, or larval, or 

 tadpole state, and being provided with an inhalent 

 and an exhalent tubular orifice, their food, which is 

 composed of microscopic organisms and sedimentary 

 matters, being drawn to the mouth by the respiratory 

 currents. The members of a large tribe, however, 

 are pelagic in their habits, being for ever in action 

 near the surface of the ocean, swimming b}^ the aid 

 of their respiratory currents, which in these instances 

 have the threefold office to perform of aiding in 

 respiration, in nutrition, and in locomotion. 



And, finally, the Tunicata are either simple or com- 

 pound simple when [each individual is] encased 

 in a separate external envelope, compound wdien the 

 test, or outer tunic, is fused or expanded so as to 

 include many individuals in one general envelope. 



Though the Tunicata have had much attention 

 bestowed on them of late years by the thoughtful 



*/ */ O 



naturalist, they have generally been much neglected 

 by, and are but little known to, the casual observer, 



