1228 



TUNICATA. 



other at right angles. The transverse vessels, 

 varying from 18 to 25 in number, are the 



Fig. 786. 



\ 



Anatomy of Pyrosoma giganteum, magnified. 



A. A portion of the common mass with animals im- 

 bedded in its substance. (After Savigny.~) a, a, a, a, 

 the liver ; b, b, b, ova in the posterior cavity ; c, ovum 

 in the substance of the common test ; d, d, d, full- 

 grown individuals ; e, e, e, undeveloped individuals. 



B. A single individual cut out of the common test, 

 with a portion of the latter surrounding it, seen from 

 below. (After Lesueur.) a, branchial or external ori- 

 fice ; b, anal or internal orifice ; c, c, delicate fibres 

 traversing the test ; d, stomach ; e, liver ; /, /, bran- 

 chiae ; g, oviduct ( ?) ; h, ovary ; i, ventral sinus ; j, 

 nerve-ganglion. 



C. The viscera of an individual Pyrosome. (After 

 Lesueur.) d, e, g, h, i, j, as in Jig. B ; k, ovum. 



largest and most distinct, and are folded back 

 on themselves at the free edges of the tissue. 

 The longitudinal vessels are from 11 to 17 in 

 number. " Nothing is more curious," says 

 Milne-Edwards*, "than the respiratory ap- 

 paratus of these little animals, when the vi- 

 bratile cilia, with which each of the branchial 

 stigmata is furnished, are simultaneously ef- 

 fecting their vorticiform movements with ra- 

 pidity and perfect harmony." 



The oesophagus is curved, and is of a 

 bright red colour. The stomach is subglo- 

 bular, yellowish, and opaque. The intestine 

 is short, and strongly bent on itself ; the anus 

 is directed backwards towards the posterior 



* Annales des Sciences Naturelles, sec. se'r. torn, 

 xii. p. 375. 1835. 



orifice. The liver is a globular body, which 

 is but slightly developed in young individuals ; 

 its postero-inferior portion is formed of several 

 sections united by a centre, around which 

 they converge, presenting the appearance of 

 a flower with many petals, or a calyx with, 

 most usually, 7, 8. or 10 divisions. The sec- 

 tions are not always equal. Their centre is 

 occupied by a somewhat solid, granular sub- 

 stance, which they more or less perfectly 

 enclose. Its colour is generally whitish, or 

 of a light pink. It lies free in a cavity hol- 

 lowed out of the test, and is attached by 

 a membranous peduncle to the stomach, 

 or rather to the intestinal loop (Jig> 786. 

 A, a, a, B and c e, e). These viscera are si- 

 tuated posteriorly to the branchial sac. By 

 their disposition they leave a free passage to 

 the water which traverses the branchial ca- 

 vity. The nerve-ganglion is present at the 

 anterior extremity of the dorsal border of the 

 branchial sac. From it there proceed filiform 

 branches towards the neck of the external 

 tubercle and in other directions. The vessel 

 or sinus that runs between the two free edges 

 of the branchiae is of considerable length, and 

 has a slight general curve. It is divided, as it 

 were, into four, towards its largest and an- 

 terior portion, or rather seems to be com- 

 posed of two contiguous vessels bent upon 

 themselves. These diminish in calibre as they 

 run backwards, and, passing into a delicate 

 filiform vessel, are lost near the stomach. 



Two arching vessels, one on either side, pass 

 from near the nerve-ganglion to the com- 

 pressed oval bodies on the anterior lateral 

 points of the branchiae, and a similar pair of 

 vessels unite these latter bodies with the 

 extremity of the loop formed by the vessel 

 that lies between the free ventral margins of 

 the branchiae. These four arched vessels form 

 therefore a circlet around the anterior ex- 

 tremity of the branchial sac (Jig. 786. B, h). 



The heart is placed at the posterior part of 

 the body, at the side of, and below, the visceral 

 mass. Its character is perfectly analogous to 

 that of the heart of other Ascidians. It con- 

 tracts with the usual peristaltic ^ movement, 

 and changes periodically the direction of this 

 vermicular movement, the vessels alternately 

 playing the part of artery and vein. 



With regard to the generation of the Pyroso- 

 midcE, very Jittle has yet been observed. Sa- 

 vigny describes as "oviduct" and "siphon- 

 canal" the vessels that occupy the usual 

 position of the dorsal sinus, running along the 

 dorsal or superior surface of the branchial 

 sac (Jig. 786. g, g). They have, however, no 

 apparent connection either with the pre- 

 sumed oviferous bodies situated at the an- 

 terior points of the branchiae, or with the cavity 

 posterior to the abdominal viscera occupied 

 by the ova in Savigny's specimens (Jig. 786. 

 A, b). Lesueur discovered and described cer- 

 tain globular, transparent bodies in Pyrosoma, 

 situated near the liver (Jig. 786. c, k), which 

 he regarded as ova ; these were noticed also 

 by Savigny. Each enclosed four minute Py- 

 rosornes, symmetrically disposed, and readily 



