RESPIRATORY ORGANS. 



355 



.-in 



(No. 17) in regarding as the homologue of the coxal gland of the 

 Scorpiones, and which is probably to be considered as a modified 

 nephridiuni, is also a derivative of the mesoderm. This paired 

 gland which, in the adult, seems to have no external aperture,* is 

 situated on either side of the endosternum, close to the coxal 

 joints of the thoracic limbs (second to fifth) ; it consists of coiled 

 anastomosing tubes, massed together, but divided into a longi- 

 tudinal body and four vertical lobes. In the young stage observed 

 by GuLLAND (No. 13), these latter are wanting, but the glands open 

 outward on the coxae of the fifth pair 

 of limbs. According to Kingsley, 

 this organ develops in the embryo 

 from the mesoderm, and includes 

 part of the coelomic cavity of the 

 fifth post-oral segment. Its inner 

 end opens out into the coelom of 

 the fiftli post-oral somite. The 

 cubical epithelium of the gland 

 passes into the pavement epithelium 

 of the coelomic cavity ; the latter 

 jepresents the end -sac. It thus 

 appears essentially to resemble in 

 structure the nephridia of Peripatits. 

 The tubular portion of the gland 

 first bends anteriorly, the outer part 

 of this coil forming four more loops. 

 These new secondary loops develop 

 in each segment into the four lobes 

 of the adult gland mentioned above. At the points where the coils 

 come into contact, fusion and perforation take place. 



D. Respiratory Organs. 

 The branchial lamellae (Fig. 165, K) arise at the posterior dorsal 

 side of three of the abdominal limbs (second to fifth) as simple out- 

 growths of the body-surface. At first they are few in number, but, 

 as development progresses, new rudiments of lamellae are continually 

 added to the basal segments of these appendages. Kingsley has 

 pointed out that, in the early stages of the development of the gills, 

 the whole region appears slightly sunk below the surface, as if 

 foreshadowing the formation of the invaginate lung-books of the 

 Scorpion (Fig. 165, go). 



* [See Tower, Ai)p. Lit. Xipliosura, No. VI. — Ed.] 



/^/ 



O 



Fro. 165. — Longitudinal section through 

 the abdominal appendages of a Limulus 

 embryo, to Illustrate the origin of the 

 branchial lamellae (after Kingsley). 

 (/«!, ga„, first and second gill-bearing 

 limbs ; o, operculum ; K, branchial 

 lamellae ; m, muscle ; d, food-yolk. 



