386 PROTOTRACHEATA. 



are two special longitudinal septa dividing the body cavity into three 

 compartments, a median (me), containing 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 division is found in 

 some Chaetopoda, 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 stomo- 

 daeum, and proctodaeum. 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 fonvard extension of the 

 mesenteron is remarkable. 



The stomodaeum 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- 

 daeum 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 

 the reach of scepticism. If the reader will compare the figure of Peripatus 

 with that of an embryo Scorpion (fig. 196 A) or Spider (fig. 200 C) or better 

 still with Metschnikoffs figure of Geophilus (No. 399) PL xxi. fig. u,he 

 will be satisfied on this point. 



The homologies of the anterior appendages are not very easy 

 to determine ; but since there does not appear to me to be suffi- 

 cient evidence to shew that any of the anterior appendages have 

 become aborted, the first post-oral appendages embedded in the 

 lips may provisionally be regarded as equivalent to the mandibles, 

 and the oral papillae to the first pair of maxillae, etc. Moseley is 

 somewhat doubtful about the homologies of the appendages, 

 and hesitates between considering the oral papillae as equivalent 

 to the second pair of maxillae (on account of their containing the 

 openings of the mucous glands, which he compares with the 

 spinning glands of caterpillars), or to the poison claws (fourth 



1 The specimens shewing tracheae 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. 



