208 



THE ACEPHALA. 



^ 192, 



intestine ; and, as with Salpa, the direction of the current is changed so»- 

 alternately that the two terminal vessels serve in rotation as an Aorta and 

 a Vena cava/"' 



The blood not only traverses the lacunae of the intestinal sac, but also 

 penetrates the walls of the mantle, and even passes into the common sup- 

 port of the compound forms. In this last case, it circulates in ramified 

 canals, which, as prolongations of the cavity of the body, extend even into 

 this portion of the mantle.'"' 



With the Brachiopoda, this system is quite remarkable. The branchial 

 afferent veins of the mantle do not open into a single heart, but into two 

 hearts which are situated right and left of the intestinal sac.***' 



These hearts, by pulsation, throw the blood into the intestinal canal, 

 which ought therefore to be considered as a common visceral sinus. '*" 



With the Lamellibranchia, the heart, situated at the posterior extremity 

 of the back, is divided, usually into three chambers, and surrounded with 

 a large pericardium. Two lateral, triangular, thick-walled auricles receive 

 the blood from the branchiae and send it into a simple muscular ventricle- 

 which is nearly always traversed by the rectum. Thence the blood passes 

 into the body by a posterior and an anterior aorta. Its return into the- 

 two auricles is prevented by valves.*^"' The walls of these aortae disappear 

 after considerable ramification, and the blood passes into a system of lacu- 

 nae which extends through the whole body and forms a net-work of sinuses 

 and anastomosing canals. '^^' The venous blood is received into special 



6 This change in the direction of the blood-cur- 

 rents was first noticed by Linter (Philos. Trans. 

 1834, Pt. II. p. 365, or fViegmann's Arch. 1835, 

 I. p. 309) with Perophora, .a new genua of the 

 compound Ascidiae ; and Milne Edwards has 

 since confirmed it with Pyrosoma (Ann. des Sc. 

 Nat. XII. 1839, p. 375), and several other Ascidiae 

 both simple and compound : see his Observ. sur 

 les Ascidies simples et composees, p. 7. 



These inter-alternating peristaltic and anti-peris- 

 taltic motions show that the heart of the Ascidiae 

 is valveless. It is therefore surprising that Delle 

 Chiaje has described it with valves ; but this is 

 not the only point in which he differs from other 

 observers on this subject, for he describes the heart 

 of the Ascidiae as bifurcated into two auricles ; see 

 his Mem. &c. loo. cit. III. p. 193, Tav. XLVI. fig. 

 13, ab. {Cynthia papillata), and Descriz. &c. III. 

 p. 29, Tav. LXXXII. fig. 11, 12 {Pkallusia in- 

 testinalis'). 



1 This circulation of the blood in the common 

 Ascidian-stock has been observed by Lister (loc. 

 cit.). Milne Edwards has seen also the ascending 

 and descending currents in the ramified and coecal 

 prolongations of the peritoneal sac, in Botrylliis, 

 Diazona, Didemtium, und Polycliniim ; see Sa- 

 vigny. Mini. loc. cit. p. 47 ; Delle Chiaje, Descriz. 

 &o. III. p. .34, Tav. L.XXXIII. fig. 13, 15 ; and 

 Milne Edwards, Sur les Ascidies, loc. cit. p. 41, 

 PI. VII. fig. 1, V>. V. This last-mentioned author 

 has also observed that, with Clavelina (Ibid, p. 

 9. PI. II. >, these canals terminate in caeca which 

 communicate with the cavity of the body, and are 

 extended into digitiform prolongations upon the ex- 

 tremity of the peritoneal sac, and herein the blood 

 moves alternately up and down. The ramified ca- 

 nals which abundantly traverse the mantle of Pkal- 

 lusia, are, according to authors, real blood-vessels ; 

 see Cuvier, loc. cit. p. 16, PI. III. fig. 1 ; Savis^ny, 

 loc. cit. p. 102, PI. IX. fig. 1, B., and Delle Chiaje, 



Descriz. &c. III. p. 33, Tav. LXXXIV. fig. 2. 

 According to Kiillitcer (Ueber das Vorkommen der 

 Holzfaser im Thierreich. loc. cit.), these multira- 

 mose vessels which come directly from the heart 

 and whose extremities are penicillated, appear to be 

 continuous directly beneath the skin with other" 

 vessels returning by the course of these arteries. 



8 See Cuvier, Owen, and Vast, loc. cit. 



9 Owen was the first to notice this analogy of 

 the circulation of the Brachiopoda with the extra 

 vascular one of other Acephala ; see his Lettre sur 

 I'Appareilde la circulation chez les Mollusquesdela 

 Classe des Brachiopodes (Ann. d. Sc. Nat. III. 1845,. 

 p. 315, PI. IV., or Froriep^s neue Not. No. 793). 



lU i^or the arrangement of ihis central part of the 

 circulatory system, see Poll, loc. cit. Tab. IX. fig. 

 12 (ü'nio)i Tab. XIll. fig. 5 (Solen); Tab. XXII. 

 fig. 10 (Spondylus); Tab. XXV'II. fig. 8, 12 (Pec- 

 ten); Tab. XXIX. fig. 7,8 (Ostrea) ; Tab. XXXI. 

 fig. 8, 9 (Mytilus), and Tab. XXXVIII. X^KIX. 

 (Pinna). Also Bojanus, in the Isis, 1819, j 42, 

 Taf. I. II. (Anodonta) ; Trevira?ius, BeobachI aus 

 d. Zoot. u. Physiol, p. 44, fig. 67 69 (Mytilus and 

 Anodonta) ; and Garner, Trans, of the Zool. lioc. 

 II. p. 90, PI. XIX. fig. 4 (Pecten). 



An arrangement quite different from this type is 

 found with Area, whose two auricles are attached 

 to the two widely-separated ventricles, and send 

 out on each side an anterior and posterior aorta, 

 which meet and join upon the dorsal median hue ; 

 see Poli, loc. cit. Taf. XXV. fig. 2, 3.* 



11 This system of lacunae forms, especially in tho 

 mantle, a beautiful net-work of delicate canals 

 which, with the Na'iailes, are visible to the naked 

 eye. It should not, however, be confounded with 

 another net-work more difiicult to be seen, and 

 which probably constitutes a system of aquiferous 

 canals, which is easily seen in the mantle, foot and 

 other parts of the body by inflation. Delle Chi- 

 aje has called it Rete lymphatico-vasculosum. 



* [ § 192, note 10.] See also Deshayes, loc. cit. p. 63, 64, &c., PI. VIII. fig. 1, 2, 3, and ^uatre- 

 Jagea, loc. cit. p. 47, PI. I. fig. 7 (Teredo). — Kd. 



