T7PE ARACHNIDA. 463 



forwards towards tlie ventral surface, on nearing whicli they 

 unite to form a long coiled tube, the uterus {ut), which passes 

 backwards to the genital orifice, and just where the two ducts 

 unite they have opening into them a pair of pyriform seminal 

 receptacles (rs). The vasa deferentia are also paired, and 

 arise at the anterior end of the testis, passing ventrally 

 towards the genital pore, uniting before they reach it and 

 dilating to form a complicated intromittent organ, from which 

 two tubes with muscular walls and containing spermatozoa 

 project backwards and serve as ejaculatory ducts for the ex- 

 pulsion of the spermatozoa through the intromittent organ. 

 The only genus belonging to the order is Pentastomum. 



Development of tJie Pentastomidm. — During the life-history of a Pen- 

 tastomum it passes through a marked metamorphosis associated with a 

 change of hosts, recalling what occurs in the Cestoda. The ova are passed 

 to the exterior with the excreta of the host, or, in the case of the dog, 

 with the mucous discharge from the nasal passages, and the embryo which 

 hatches out is a decidedly Mitelike form, possessing, however, only two 

 pairs of legs terminating in ungues. No other appendages are present, 

 but the embryo is provided anteriorly with a boring apparatus. If this 

 larva of P. tcenioides, the parasite of the dog, succeeds in gaining entrance 

 to the digestive tract of a rabbit or cat, for instance, it bores through the 

 wall of the intestine and, reaching the liver, encysts itself. Within the 

 cyst it undergoes several moults, finally assuming a condition similar to 

 the adult except that each annulus bears a circle of hooks. Leaving the 

 cyst, then, it wanders through the tissues of the host, and if while it is in 

 this condition the host is eaten by a dog, it adheres to the mucous mem- 

 brane of the mouth of the latter, and makes its way into the nasal passages, 

 there moulting again, losing the ring of hooks and assuming the adult form. 



The principal reason for supposing Pentastomum to be related to the 

 Arachnids is the occurrence of the four-legged larva, which resembles, so 

 far as its external form is concerned, a Mite. The internal structure is 

 very different, however, although certain Arachnid features are indicated ; 

 but it is evident that these forms must have undergone an enormous de- 

 parture from the ancestral form during which the remarkable life-history 

 and peculiar structure have been acquired. The parasitic habits of many 

 Mites, and the general similarity of the body form of Demodex to that of 

 Pentastomum, suggest the Mites as the ancestors of the latter, a theory 

 which is as plausible as any other which can at present be suggested. 



Order Fycuogonida. 

 The Pycnogonida are exclusively marine in habitat, and 

 vary considerably in size, the smaller forms, such as Tanysty- 



