T RAG HE AT A. 



319 



sp.m 



FIG. 172. SECTION THEOUGH THE TRUNK 

 ^ EMBRYO OF PEKIPATUS. The embryo 



Similar epiblastic invaginations assist in forming the supra-cesophageal 

 ganglia of other Tracheata. They are described in the sequel for Insects, 

 Spiders and Scorpions. The position of the supra-oasophageal ganglia on 

 the ventral side of the procephalic lobes is the same as that in other 

 Tracheata. 



The mesoblast is formed, in the 

 earliest of my embryos, of scatter- 

 ed cells in the fairly wide space 

 between the mesenteron and the 

 epiblast. There are two distinct 

 bands of mesoblast on the outer 

 sides of the nervous cords. In the 

 later stage the mesoblast is divided Am ' 

 into distinct somatic and splanch- 

 nic layers, both very thin ; but 

 the two layers are connected by 

 transverse strands (fig. 172). There 

 are two special longitudinal septa 

 dividing the body cavity into three 

 compartments, a median (me), con- OF 

 taining the mesenteron, and two 

 lateral (Ic) containing the nerve 

 cords. This division of the body 

 cavity persists, as I have elsewhere 

 shewn, in the adult. A similar di- 

 vision is found in some Chsetopoda, 

 e.g. Polygordius. 



I failed to make out that the mesoblast was divided into somites, and 

 feel fairly confident that it is not so in the stages I have investigated. 



There is a section of the body cavity in the limbs as in embryo Myria- 

 pods, Spiders, etc. 



In the procephalic lobe there is a well-developed section of the body 

 cavity, which lies dorsal to and in front of the rudiment of the supra- 

 cesophageal ganglia. 



The alimentary tract is formed of a mesenteron (fig. 172), a storno- 

 dseum, and proctodseum. The wall of the mesenteron is formed, in the 

 stages investigated by me, of a single layer of cells with yolk particles, 

 and encloses a lumen free from yolk. The forward extension of the 

 mesenteron is remarkable. 



The stomodseuru in the earlier stage is a simple pit, which meets but does 

 not open into the mesenteron. In the later stage the external opening of 

 the pit is complicated by the structures already described. The procto- 

 dseum is a moderately deep pit near the hinder end of the body. 



The existence of a tracheal system 1 is in itself almost sufficient to 

 demonstrate the affinities of Peripatus with the Tracheata, in spite of the 

 presence of nephridia. The embryological characters of the procephalic 

 lobes, of the limbs and claws, place however this conclusion beyond 



informs me that he has long been aware of the homology of these folds with those in 

 various other Tracheata. 



1 The specimens shewing tracheas which Moseley has placed in my hands are quite 

 sufficient to leave no doubt whatever in my mind as to the general accuracy of his 

 description of the tracheal system . 



sp.m. splanchnic mesoblast. 



s.m. somatic mesoblast. 



me. median section of body cavity. 



7c. lateral section of body cavity. 



v.n. ventral nerve cord. 



me. mesenteron. 



