212 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



is impregnated with haemoglobin {see Power's 

 " Human Physiology,"), which has a remarkable 

 affinity for oxygen. This haemoglobin is either found in 

 special corpuscles, as in the Vertebrata, some Annelids 

 (Glycera, Capitella), some Gephyreans (Phoronis, 

 Thalassema, Hamingia), some Lamellibranchs (Solen, 

 Area), or it is diffused in the liquid of the blood, as in 

 most, though not all, Polychsetous and Oligochaetous 

 Annelids and Hirudinea, some Nemertines, some 

 Crustacea, the mollusc Planorbis, and the larva of the 

 insect Chironomus.* It has been found in a special 

 system of vessels in the Crustacean Lernanthropus and 

 Clavella, and in the corpuscles of the water vascular 

 system of Ophiactis virens. In various forms it is 

 found diffused in muscular tissue. Its most remark- 

 able position, however, is in the nerve tissue ; this has 

 been observed in the sea-mouse (Aphrodite aculeata) 

 and in a number of Nemertinea ; in the latter it is 

 absent from the nerve-tissue of those forms in which 

 the blood is impregnated with it. 



Among the higher worms in which no special 

 respiratory apparatus is developed, it will probably be 

 frequently observed, as has already been the case with 

 the leech, that the epithelial covering of the body is 

 interpenetrated by capillaries (Fig. 91); " the true 

 respiratory organ of the leech is clearly this vascular 

 epiderm, and amongst respiratory organs it stands 

 alone in the nearness with which the absorbent blood- 

 vessels succeed in bringing themselves through all 

 structural obstacles into direct contact with the 

 oxygenating medium " (Lankester). 



When specialised respiratory organs are developed 

 they may arise in various ways, either as in-pushings 

 of the surface, which may become slits or tubes, or as 



* The larger number of these cases have been observed by Ray 

 Lankester. See the Proceedings of the Royal Society, 1873, No. 

 140. 



