iv VERMES RESPIRATORY ORGANS 247 



the parapodial cirri ; they may occur simultaneously on the ventral 

 and dorsal parapodia. They are found in the Capitellidce and the 

 Glyceridce, and in the absence of a separate blood-vascular system are 

 provided with continuations of the body cavity carrying haemolymph. 

 The blood gills, on the contrary, are often branched appendages of 

 the parapodia which are penetrated throughout by blood-vessels. We 

 can again distinguish two sorts of blood gills dorsal gills and cephalic 

 gills. The dorsal gills are transformed cirri of the dorsal parapodia, or 

 transformed lateral off-shoots of such cirri. They occur, like the para- 

 podia themselves, in segmental order, but may often attain development 

 only in certain regions of the body (branchial regions). The cephalic 

 gills, on the other hand, which we meet with specially in tubicolous 

 Annelids, are transformed tentacles or feeler-cirri of the head, and often 

 form a beautiful crown of gills or tentacles projecting above the aperture 

 of the tube. These crowns are at the same time also the seat of a fine 

 sense of touch, and, further, organs for the drawing in of nourishment. 

 In the Sabellidce, (Branchiomma) eyes may be developed on the cephalic 

 gills. Where gills attain development they are almost always either 

 ciliated or mobile, so that a constant exchange of the respirable 

 medium, oxygenated water, is secured. In the thoracic membrane of 

 the Serpulidce there is a rich network of vessels, and this no doubt has 

 a respiratory significance. In Sternaspis, on each side of the anus, there 

 is a tuft of filamentous gills. We do not find special respiratory organs 

 in the Myzostomidce, the Chcetognatha, and the Rotatoiia, in whom a blood- 

 vascular system also is wanting. In all these forms cutaneous respira- 

 tion must take place, facilitated in the wheel animalculae by the activity 

 of the wheel organ. The cutaneous respiration in the Echmridce is 

 principally localised in the prostomium. In the Prosopygia the tentacles 

 which stand round the mouth, or the oral arms furnished with cirri 

 (Brachiopoda) chiefly act as gills, and these organs are either penetrated 

 by blood-vessels (Phoronis, Brachiopoda?) or supplied with canal or 

 vessel-like processes of the body cavity (Sipunculidce, Bryozoa). In the 

 Brachiopoda the inner surface of the mantle has in all cases an addi- 

 tional respiratory significance. The Priapulidce among the Sipunculacea 

 possess no oral tentacles; in them respiration takes place through the 

 integument. Besides this, however, there can be little doubt that 

 in Priapulus the deeply-lobed caudal appendage into which, in the 

 absence of a blood- vascular system, a continuation of the body cavity 

 extends, may be regarded as a respiratory organ. 



Just as the cutaneous respiration may be concentrated in certain 

 localised parts of the integument, in which places the principle of in- 

 increase of surface is applied, accompanied by a richer vascularisation, 

 the enteric respiration also may be localised. The accessory intestine 

 which is found in some Polyclmta and Sipunculacea is said to represent 

 such a respiratory portion of the enteric canal. Respiratory organs 

 are wanting in the Nemathelminths. 



