AND DEVELOPMENT OF PYROSOMA. 209 



funnel-shaped bag, about xootb of an inch long, which lies in the sinus, and, passing 

 obliquely forwards and towards the hsemal side, opens as above described. Its aperture 

 has somewhat prominent lips, and is rather narrower than its upper portion. The poste- 

 rior end of the sac appears to terminate caecally, and is applied against the posterior sur- 

 face of the ganglion. The middle of the haemal side of the sac sometimes appears to be 

 connected with a spheroidal tubercle, whose axis forms nearly a right angle with that of 

 the sac. 



The muscular system is exceedingly simple in this species of Fyrosoma, consisting, 

 besides the oral sphincter and buccal muscles already mentioned, of only an atrial 

 sphincter and the ' mid-atrial ' muscles. 



The atrial aperture (fig. 7) is even smaller than the oral, not measuring more than from 

 ■g^th to y^th of an inch in diameter. Radiating striae diverge from its margin on the 

 surface of the test, which, as at the oral aperture, forms a thick lip, and is continued for 

 some little distance inwards upon the wall of the mid-atrium. A sphincter formed of pale 

 smooth fibres, and constituting a circular band xe^tli of an inch in diameter, is developed 

 at the junction of the external and atrial tunics. There is a similar but less distinct 

 appearance of radiating fibres to that exhibited at the oral sphincter. 



The mid-atrial muscles {g 2) are broad flat bands of smooth muscular fibres, which lie in 

 close contact with, and apparently attached to, the atrial tunic. One of these bands 

 occupies about the middle two-fourths of the height of each lateral wall of the mid-atrium, 

 and has a direction perpendicular to the axis of the ascidiozooid. The bundle of fibres 

 spreads out a little at each end, and then seems to be inserted by a sort of tendon into the 

 outer tunic. Close to this tendinous insertion, at either end, a bundle of fibres (whether 

 merely fibrous or muscular I cannot say) arises, and passes, partly to the nearest similar 

 insertion of one of the mid-atrial muscles of the ascidiozooid above or below, partly to the 

 same point of the mid-atrial muscle of some other ascidiozooid. Hence, when the wall of 

 the ascidiarium is viewed from within, it presents such an interlacement of fibres as that 

 exhibited in fig. 9. 



These muscles, in contracting, must tend to diminish the capacity of the atrium of the 

 ascidiozooid to which they belong, and, if they all act together, to shorten and narrow the 

 ascidiarium. I do not suppose that their effect in the latter direction can be very great ; 

 but it might well be sufficient to account for the slight contraction of the whole ascidi- 

 arium, and consequent retrogressive motion, observed by Peron and others. 



In my previous memoir, I have pointed out that the round, granular, yellowish patches 

 on each side of the entrance of the branchial sac, and opposite the middle of the peri- 

 pharyngeal ridge, are not, as Savigny imagined, the ovaries. I am greatly inclined to 

 regard them as renal organs, but for the present defer the discussion of their structure 

 and functions. 



The reproductive organs of each ascidiozooid of Fijrosoma may be divided into actual 

 and potential — the genitalia of the ascidiozooid itself, and the blastema whence the 

 genitalia of its buds will take their origin. I shall call this last the generative blastema ; 

 while the genitalia proper are divisible, as I have already pointed out, into a single ovisac 

 and a single testis. Both ovisac and testis are situated in the left five-sixths of the roof 



VOL. XXIII. 2 r 



