*§><§> 193, 194. THE ACEPHALA. 209 



lacunae situated at the base of the branchiae, and into which it thence 

 passes. 



CHAPTER Vir. 



EESPIEATORY SYSTEM. 



§ 193. 



With all the Acephala, the blood, just before returning to the heart, 

 passes through a branchial, or distinctly respiratory organ, which, either 

 simple or multiple in structure, is always hidden in the cavity (5f the man- 

 tle. The renewal of water takes place by special openings of the body, or 

 through the slits of the mantle, which are often prolonged into two respir- 

 atory tubes. One of these openings is for the ingress, and the other for 

 the egress of the water, and their currents carry in and out, respectively, 

 food and faeces. In the cavity of the mantle, the water circulates in a 

 definite direction and passes over the branchiae by means of the cilia cover- 

 ing their external surface. 



§194. 



The Branchiae of the Acephala are formed after four different types : — 

 1. With Salpa, there is one only of these organs which stretches, from 

 above downwards and from before backwards, across the cavity of the body. 

 The water enters through an anterior orifice which is usually valvular, and is 

 expelled through a posterior opening by the contractions of the body.*'' 

 The branchia itself, which, near the heart, is bent a little in front at its 

 lower posterior extremity, consists of a narrow band having upon one of 

 its sides numerous transverse, thickly-set folds.*-'. Its remaining portion is 

 flat,® or the lateral borders are rolled up like tubes.**' The branchial ves- 

 sels are ramified in the interior, communicating, at the superior extremity 



and has figured it very beautifully ; see his Des- posterior orifice, the animal closes the valve of the 



criz. &c. Tav. LXXV. fig. 6, and Tab. XC. fig. 1, anterior one, so that the body is thrown forwards. 



2 (Mantle of Pecten and Solen), Tav. LXXXIX. On this account the cavity of the body ia often 



fig. 11 (foot of a Mactra). called natatory. 



The vascular net-work which Poll (loc. cit. Tab. - See Cuvier, and Savigny^ loc. cit. 



XXXVIII.) has figured in the mantle of a Pinna, 3 Salpa cantata, and maxima. 



ia probably only one of aquiferous canals. 4 Sa/pa pinnata, cylindrica, octofora. When 



I shall again (§195) allude to this confusion the brancliia is contained in a tube it has often been 



between the blood and aquiferous vessels.* compared to a Trachea ; see Savigny, loc. cit. PI. 



1 These respu-atory motions aid also for the lo- XXIV. 

 comotion of Salpa ; for, when water escapes by the 



* [§ 192, note 11.] This lacunal system is well- heart, and on the other hand, all the other lacunae 



marked with Teredo according to (^uatrefages, and the arterial system beside may be filled by in- 



who denies that these animals have a proper venous jecting through one lacuna ; see M^moire, loc. cit. 



system. The grounds of this conclusion are, that p. 55. — Ed. 

 tln-'se lacunae are always filled by injecting the 



18^ . 



