602 



COMPAEATIVE ANATOMY. 



region of the anus, while anteriorly it opens into the coelom by an 

 internal orifice. Although it is clear that this arrangement has much 

 in common with the excretory organs of the Vermes, yet the pecu- 

 liarity must not be overlooked, that although the vertebrate body is a 

 metam eric one, this archinephric duct is not a metamerised organ; 

 it is not therefore completely homologous with the metameric looped 

 canals of the Annulate Vermes. It must consequently be derived 

 from a still lower condition, that is, from one in which the organism 

 was not divided into metameres ; so that it represents, as does also 

 the unsegmented chorda dorsalis, one of the, phylogenetically, oldest 

 organs of the Vertebrate body. 



This archinephric duct has been observed to be derived from the 



mesoderm ; in its rudimentary condition 

 it has the form of a solid chord of cells, 

 or is differentiated as a groove from 

 the epithelium of the peritoneal cavity 

 (Teleostei). The rudiments of the canals 

 (Fig. 342, t) are derived from the same 

 parts ; these, either permanently, or for 

 a time only, open by an infundibular 

 orifice into the abdominal cavity, while 

 they are also connected with the archi- 

 nephric duct (Selachii, Amphibia). They 

 develop coiled glands on their course, 

 and so form the secreting portion of the 

 primitive kidneys. At definite points 

 j^^ a coil of arterial vessels (glomerulus) 

 \^Jj pushes its way into a dilatation of these 

 ^v^=-^ metamerically arranged canals, and gives 

 rise to a Malpighian body, lying in a cap- 

 sular enlargement. This last arrangement 

 obtains in all forms of the renal organ, 

 however much it may be modified in 

 various members of the vertebrate group. 

 The fundamental form' of this primi- 

 tive kidney must be regarded as being a 

 longitudinal canal, which receives trans- 

 verse canaliculi, which open by ciliated 

 inf undibula into the abdominal cavity ; this is the form which the 

 rudimentary apparatus really has in the Selachii. The connection 

 with the coelom, the epithelial investment of which always gives 

 rise to a large portion of this system, allows us to compare it with 

 the excretory organs of many Vermes, and points to those distant 

 forms in which these organs are the sole cavitary organs that are 

 developed from the mesoderm (Platyhelminthes). The metameric 

 arrangement of the open transverse canals is due to the general 

 metamerism of the vertebrate organism. It must not, therefore, be 

 regarded as the same as that of the looped canals of the Annelids, or 

 even as derived from it, for those canals open to the exterior on the 



Fig. 342. Section of an Embryo 

 of Pristiurus. ug Archi- 

 nephric duct, t Eadiment of a 

 funnel-shaped organ. dEnteron. 

 VI Medullary tube, ch Noto- 

 chord. a Aorta, v Veins. 



