318 ME. W. B. S. BENHAM ON THE MUSCULAR 



Thus there are six of these abdominal entochondrites, and six pairs of tendinous 

 stigmata. 



The sides of the mesosoma rise up, and are continuous with a chitinous portion, 

 which continues outwards, and becomes fused with the tergite ; the two thus fused are 

 ccmtinued laterally for a short distance, then separate again and thicken out, containing 

 a canal, carrying an artery &c. to the movable abdominal spines. The floor of this is 

 horizontal. 



The membranous floor of the mesosoma is continuous behind with the chitinous floor 

 of the metasoma ; this is scooped out on its anterior border, in tlie middle line ; in the 

 hollow thus formed is situated the last entochondrite, and to the sides of this hollow 

 are attached the last pair of tendinous stigmata (see PI. LXXII.). The metasomatic 

 floor itself bends sharply downwards, widens posteriorly, and curves upwards at tlie 

 sides to join the tergite ; thus it is concave from within. This metasomatic cavity is 

 continuous with the lateral canal above mentioned. 



The hinder border, which is almost flat, is scooped out ; and in this bay is situated 

 the anus, surrounded by a membrane similar to that round the mouth (R). Behind this 

 is the postanal spine (sp). 



Thus, if the abdominal region be looked at from below, supposing the appendages to 

 be removed, the sides curve upwards towards the observer (downwards, of course, in its 

 natural position), and outside this is the flattened floor of the lateral canal 



On the concave sides are five transverse lines (see woodcut, fig. 3, in Lankester's 

 " Limulus an Arachnid "), corresponding with those slight depressions seen on the 

 abdominal tergite, starting from between each pair of entapophyses. From the last 

 line rises upwards (downwards in natural position) the metasomatic sternite. This line 

 starts between the sixth and seventli entapophyses, so that the latter lies in the meta- 

 soma, and, as will be seen by the muscles attached to it, must be considered as belonging 

 to this portion. In the same way the muscles attached to the first pair of entapo- 

 physes, which are invaginated from the posterior vertical border of the prosomatic 

 carapace, seem to show that these belong really to the mesosoma. 



The microscopical strncture of the carapace shows it to consist of three layers of 

 chitin of various thicknesses, the outermost being very thin and remaining yellow, while 

 the second remains almost colourless, and the innermost deeply stained under the 

 action of borax-carmine. The middle layer shows fine wavy lines parallel to the surface, 

 as well as finer transverse striations. The inner layer is more coarsely striated, mainly 

 transversely, but sometimes obliquely, to the surface. 



These layers are traversed by fine tubes, which on reaching the outer layer contract 

 suddenly into an exceedingly fine capillary; these contain connective tissues, and to 

 some are attached hairs, around whose bases the external layer is depressed into a 

 small pit. 



