REPORT ON THE TUNICATA. 



127 



are clearly in the direction of more perfect adaptation to the conditions of a free- 

 swimming pelagic existence. 



fen.. I ^■•^- '^^■ 



i\ 



k. 



i^l. 



to' Trf m^i in- m" vf. \ m." m' 



m /\ at. 



z.- 



at.in. 



■n l{ \hs. 



6e. I 



Fig. 16. — Semi-diagramm.itic representation of Sa!pa from the left side. 



a. anus ; at. atrial ajierture ; at.m. muscles of atrial aperture ; Ir. branchial aperture ; br.m. muscles of branchial 

 aperture; br.s. branchial sac; d.l. dorsal lamina ( = "giir'); d.t. dorsal tubercle; end. endostyle ; h. heart; 

 i. intestine ; /. languet ; m. mantle ; iii^ — wi'", muscle bands ; n.g. nerve ganglion ; ft', oesophagus ; ov. embryo in 

 ovisac ; p.br. peribr.anchial cavity ; p.p. peripharyngeal band ; st. stomach ; s.gl. subneural gland ; t. test ; 

 t.' thickened test over viscera ; tes. testis ; z. zona prffibranchialis. 



The very remarkable Octacnemus hythius, described first by Moseley from a 

 Challenger specimen, is probably an abnormal and degenerate form allied to Salpa, 

 which has migrated into deep water and become fixed, undergoing at the same time 

 certain changes in body-form and in the arrangement of the musculature. The viscera, 

 however, still form a "nucleus" as in the tj^icskl Salpie (compare Figs. 16 and 17). 



aljn 



Fig. 17. — Diagram showing the probable structure of Octacnemus. (From left side.) 

 ad. probable place of attachment; at. atrial aperture; at.m. membrane lining the peribranchisil cavity; 

 br. branchial aperture ; hr.s. branchial sac : end. end' portions of endostyle ; i. intestine ; m. mantle ; n.g. nerve 

 gangUon ; a. oesophagus ; m: ovary and testis ; p.hr. peribranchial cavity ; St. stomach ; t. test. 



The side walls of the branchial sac in Octacnemus have not jjecome aborted, and the 

 stigmata have apparently closed up. 



Returning now to the ancestral Appendiculariidse close to Ap2)enclicularia mossi 

 {table, p. 120), it is found that in the second great ancestral line diverging from this 



