THE ANATOMY. RESPIRATORY ORGANS 81 



of accounting for its existence vanishes also ; but this is, of course, purely 

 speculative. 



There are various interesting analogies, if no more, between this series of glands 

 in the Oligochaeta and certain structures in the Vertebrata. Weldon has traced 

 the origin of the supra-renal body in the Marsipobranch Bdellostoma to a detached 

 portion of the pronephros, and he has described and figured an originally secreting 

 gland with blood in its lumina — a state of affairs closely paralleled by the condition 

 which I have just described in Evdriloides. A closer resemblance still is offered by 

 such glands as the thyroid and thymus; originally connected with the oesophagus, 

 or with diverticula of the same, these glands become entirely detached from it, and 

 have acquired some function in relation to the vascular system. It is also said 

 that the spleen is developed as a diverticulum of the gut ; if this be proved to be 

 the case the analogy is so close that we are almost justified in an actual comparison 

 of the blood-glands of these Oligochaeta with the vertebrate spleen. The spleen is 

 permeated by blood-vessels just as are the glands in the Oligochaeta, and its origin 

 as a diverticulum of the mesenteron leaves no resemblance unaccounted for. At 

 the very least the analogy is interesting and I draw attention to the resemblances 

 for what they may be worth. 



VIII. Respikatoey Organs. 



DeaUng as this woi-k does with a group of worms which were termed by Cuviek 

 ' Anndlides s^tigferes abranches,' and which are constantly distinguished by the absence 

 of branchiae from the Polychaeta the present chapter might seem to be superfluous. 

 Nevertheless there are a few forms which possess special respiratory organs, and 

 two in which these branchiae are much like those of certain Polychaeta. In the 

 great majority of Oligochaeta there are no special respiratory organs— the generg,! 

 body surface occupying the place of a lung or branchia^. Where the integument is 

 thick there are always plexuses of blood capillaries in the integument, which bring 

 the vascular system into close relations with the external medium and presumably 

 allow of an exchange of gases. The blood, as has been already mentioned, is in 

 all Oligochaeta, with the exception of Aeolosoma, Chaetogaster, and certain of the 

 Enchytraeidae, tinged with Haemoglobin; we may fairly suppose that this substance 

 plays the same part in respiration among these Annelids as in the Vertebrata. The 

 efficiency of the skin as an organ of respiration in many Leeches is largely increased 



' The nephridia were once regarded as respiratory organs and as the equivalents of the tracheae of 

 insects. The dorsal pores also were considered to perform a similar function. 



M 



