331 



MANUAL OF THE MOLLUSCA. 



opinion 



has been 



ground with anatomists that not only the tunicaries, 



but the bri/ozoa, (or Ascidian Zoophytes of Dr. Johnston) should be regarded 

 a5 moUusca ; this view was recommended by Prof. Forbes, though not adopted 

 by him, and is advocated by Prof. Allman and Mr. Huxley. 



Those who have only seen the horn-coloured sea -weeds such as Flustra 

 and Notamla, drifted by the wind on the sea-beach, may have admired their 

 minute laoc-work or chain- like cells, without once dreaming they were ex- 

 amining compound animals — shell-fish, anatomically considered. But the 

 minute polypes which studded these zoophytes when alive, were undoubtedly 

 as active, and in some respects as hi^uhly organized as the lower mollusca. 

 The question is \fhether their organization is of the same kind, or type, as the 

 moUuscau, and in this respect their claims are nearly on a parallel with those 

 of the Tanicata. The relation of the Iryozoa is to the Terebratulae, as 

 shown in their oral apparatus and muscular system [Hancock), but they have 

 neither heart, arteries or veins, and the nutrient fluid is contained in the com- 

 mon visceral cavity. The ciliated gemmules of the bryozoa are not, however, 

 more unlike moUuscan larvse* than are the tadpole- shaped fry of the 

 tunicaries. 



Before proceeding further with the description of the tunicaries, we are 

 glad to avail ourselves of a diagram by Mr. Huxley, which will make it more 

 intelligible. 



225, Longitudinal, 226, Transverse section, 



in. inhalent orifice; ex. exhalent orifice; /;. branchial sac; c. atrium ("thoracic 

 chamber" of Mihie Edwards) ; o, tentacular filaments ; (j, nerve ganglion and auditory 

 vesicle ; d, thoracic vessel, (hypo-pharyngeal band); v v\ great vascular sinuses ; t', test ; 

 muscular mantle ; e, endostyle; s, stomach ; a, intestine; A, position of heart. The 

 shading isaccidently omitted on a small portion of the test by the letter g; the branchial 

 sac {b) is connected with the wall of the atrium by {hranchio-parietal) vessels crossing 

 the cavity c, c. 



* The embryo oi antiopa (p. 136) is bell-shaped at first, with a fringe of long cilia 

 round the rim which afterwards becon.es the two-lobed vehim. 



