528 THE ANIMAL KINGDOM. 



the ducts are double, and open separately on the posterior aspect of the 

 operculum or first pair of abdominal appendages. The Scorpion has a 

 copulatory organ at the end of each vas deferens, whilst in Limulns 

 the spermatozoa are shed into the water. The male Spider retains the 

 sperm in a special receptacle developed on the inner aspect of the last 

 joint of each palpus. This structure consists of a chitinoid protecting 

 envelope which often assumes complicated forms, open at the apex and 

 lodging a coiled tube. From this apparatus the sperm is transferred to 

 the female receptacula. Accessory glands, vesiculae seminales*, and re- 

 ceptacula seminis are often found except in Limulus. The receptacula 

 seminis (rarely single) possess in most Araneidae independent openings 

 in front of the female genital aperture. 



Phrynus (Pedipalpi) and the Scorpionidae are viviparous \ The 

 Tardigrada lay their ova in their cast-off skin ; the Araneidae as a rule 

 and the Pseudoscorpionidae carry their ova about attached to the abdomen. 

 The ovum of the Scorpions is telolecithal ; of others centrolecithal. The 

 mode of segmentation in the latter case varies. The young Arachnid is 

 as a rule hatched in a form resembling the adult. The Acarina however 

 are often at first hexapodous ; and in some instances the young octopod 

 animal differs from the adult in colour, presence of hairs, &c., and passes 

 through an inert stage 2 . The embryo Pentastomum taenioides lives en- 

 cysted in the liver or lungs of the rabbit : it moults and acquires two 

 pairs of hooks and a ringed abdomen, and then wanders about in 

 its host, and may become encysted a second time. When devoured 

 by the dog or wolf it becomes adult in the nasal fossae and frontal 

 sinuses. 



Certain Acarina, e. g. Demodex, are parasitic, as are the Linguatulina. 

 The Phalangidae and Solifugae are nocturnal. The Araneidae are 

 terrestrial for the most part. Some Acarina are aquatic, Limuhis marine, 

 as were the extinct Eurypterina and Trilobita. A Scorpion has been 

 found in Silurian strata, and a Phalangid in the Solenhofen slates (Meso- 

 zoic) ; Spiders occur in amber. A Xiphosurari is found in the upper 

 Silurian, Limulus in the Solenhofen slates : the Eurypterina extend from 

 the upper Silurian to the Lower Carboniferous period ; the Trilobita from 

 Cambrian to the same period, but are chiefly Cambrian and Silurian. 



1 In Sphaerogyna wntricosa (Acarina} development is intra-uterine, and the female gives birth to 

 adult males and females, which are fecundated at birth ; Laboulbene and Megnin, Journal de 1'Anat. 

 et Physiol. xxi. 1885. 



2 This stage known as Hypopus appears to be the heteromorphous nymph or immature octopod 

 form of Tyroglyphus and some allied genera. It has typically a large covering carapace ; the 

 fourth pair of limbs armed with long setae ; mouth-parts rudimentary ; the ventral integument soft 

 and furnished with suckers, especially at the posterior end of the body. The special use of this 

 stage, which passes by an ecdysis into the ordinary adult, appears to be the transport and dispersal 

 of the species. See Michael, J. L. S. xvii. 1884, and Journal Roy. Micr. Soc. (2), v. 1885, p. 22 

 et seqq. 



