432 INVEBTBBBATE MORPHOLOGY. 



number of separate invaginations of the ectoderm, the sides 

 of the retinular cells which secrete the rhabdom being in 

 reality those sides which before invagination were at the 

 surface of the body, and the rhabdom may therefore be 

 regarded as composed of portions of the general cuticle which 

 have been separated by the invagination. 



On the under surface of the carapace in the median line 

 in front of the chelicerse is a small tubercle (Fig. 195, ol) 

 which contains an organ supposed to be olfactory in function, 

 and probably some of the setae upon the basal joints of the 

 limbs may also possess a similar function. 



Nephridia are represented by a single pair of large 

 reddish bodies lying at the sides of the cephalothorax. They 

 have no communication with the exterior in the adult, but in 

 the early stages of development open upon the basal joint of 

 the fifth appendage, and are at first tubular organs and 

 nephridialike, later becoming much contorted and complex. 

 What their function in the adult may be is uncertain, and to 

 avoid possible misconceptions it seems preferable to speak 

 of them as coxal glands, a term indicating their original point 

 of opening on the basal joints (coxae) of one of the pairs of 

 limbs. 



The Xiphosura are bisexual, the genital ducts opening on 

 both males and females on the posterior surface of the oper- 

 culum near its base. The ovaries are much branched paired 

 structures, the various branches frequently anastomosing 

 even across the median line. The testes are numerous 

 spherical bodies scattered through the body and situated on 

 branching and anastomosing vasa deferentia. 



Development awi Affinities of the Xiphosura. — When the 

 young Limulus leaves the egg it presents a remarkable resem- 

 blance to a Trilobite and suggests a possible afiinity with 

 these forms which are known to occur only in the Paleozoic 

 rocks. In these same rocks there occur also the I'emains of 

 forms known as the Eurypteridte which seem to have been 

 even more nearly related to Limulus than were the Trilobites. 

 In them the cephalothorax bore apparently only six pairs of 

 appendages which resembled more or less closely those of 

 Limulus, except that the sixth pair was broad and oarlike 



