A 



MANUAL OF THE MOLLUSCA, 



PART III. 



CLAS-S VI. TUNICATA, Lamarck. 

 (Order Hetero-braticJiiata, Blainville. ) 



The lowest order of Acephalous Mollusca are called Tunicaries, being 

 protected by an elastic tunic in place of a shell. They are extremely unlike 

 shell -fish in appearance, and are denied a place in most works on conchology ; 

 having no hard skeleton they neither furnish objects for the cabinet of the 

 collector, nor materials for the speculations of the geologist.* 



Many of the Tunicaries are curious objects when seen fresh from the sea ; 

 or still better when living in those miniature aquaria, which — thanks to 

 Mr. Gosse— are now so popular.f The transparent sorts are beautiful even 

 when preserved in spirits. To the naturalist they present many points of in- 

 West unknown amongst the other mollusca, for here he meets with com- 

 pound animals, and the phenomenon of alternate generation ; they afford 

 excellent illustrations of the structure of the breathing-organ and mechanism 

 of aquatic respiration ; and they also exhibit the simplest form and condition 

 of the vascular system, in which the blood no longer circulates in one un- 

 varying direction, but ebbs and flows like the tides. |. (pp. 31, 49.) 



The principal forms of tunicated mollusca are given in plate 24, and 

 the woodcut (fig. 224) represents one of the largest and simplest kind, which 

 is drawn as if it were transparent, so as to shew the whole of its internal 

 structure. These large solitary tunicaries are termed Ascidians, from their 



* Konig supposed the S';)/jcrero??»/es to be tunicaries allied to BoUenia ; they are 

 globular bodies, -with a tessellated surface and two orifices, found in the Silurian 

 strata, and belong to the order Cystideae amongst the Ecltinodermata. The genus 

 Eschadites of Konig waN also supposed to be a fossil tunicary ; its nature is still pro- 

 blematical. See Murchison's " Siluria." 



+ At the gardens of the London Zoological Society there are examples of Ascidium 

 and Cynthia, the compound and starlike Bolryllus (pi. 24. fig. 8) and a delicate little 

 pearly C/aueZ/rwa, whose presence was first detected by Mr. Tennent the IntelligeHt 

 and obliging keeper of the aquarium. 



t In Appendicularia Mr. Huxley finds no reversal of the current. 



