J. E. GRAY. 565 



(c.) Protector or shell none. Clio, Pneumoderma : a 

 distinct order ? 

 Order II. Dactyliobranchia. Head none distinct : man- 

 tle two-leaved : branchia lamellar, in a ring on the 

 lower side of the body, between the leaves of the 

 mantle, opposite to the lateral slits : shell sub-two- 

 valved : hind part soldered, perforated, sides slit, 

 front open. Gen. Hyalea. 



Class V. SACcoPHORA. = Tnnicata, Lamk. 



Head, eyes, foot, arms or fins, none distinct : mantle soft, 

 sac-like, double, one enclosing the other ; with two aper- 

 tures, one for respiration, the other for digestion : branchia 

 lamellar, covering the whole or part of the parietes of the 

 branchial cavity : mouth at the bottom of the branchial 

 cavity, furnished with labial tentacula : hermaphrodite : pro- 

 tectors none. Marine, breathing water. 

 Order I. Holobranchia. Fixed to other bodies. Mantle 

 and tunic separate, except at the apertures : 

 branchial lamellae large, uneven, contiguous at the 

 sides, covering the parietes of the branchial cavity : 

 branchial cavity with only one aperture ; aperture 

 surrounded on the inside with a circle of filaments, 

 not opposite to, nor communicating directly with, 

 the anal opening. 

 («.) Simple. Gen. Boltenia, Phallusia. 

 (b.) Compound. Distoma, Synoicum, Euccelium. 

 Order II. Tonobranchia. Free, floating about. Shell 

 and tunic separate from the mantle, except at the 

 apertures : branchial lamellae large, uneven, not 

 contiguous at the sides, nearly covering the parietes 

 of the branchial cavity : branchial and anal opening 

 diametrically opposite : branchial cavity open at 

 both ends ; anterior opening furnished with a den- 

 ticulated ring : fringes none. (Compound.) Gen. 

 Pyrosoma. 

 Order III. Dipiiyllobranciiia. Free, floating about ; 

 shell or tunic adhering to the mantle in all parts : 

 branchial lamellae two, narrow, united : branchial 

 orifice with a valve. Gen. Salpa. 



Class VI. CoNCHOPiiORA.=Acephala testacea, Cuv. 



Head none distinct : foot one, compressed : mantle '2- 

 leaved, one on each side of the foot, more or less soldered 

 together : branchial lamellae 2 on each side, placed between 

 the foot and the mantle leaves : mouth just before the foot, 



