336 THE ANATOMY OF INVERTEBRATED ANIMALS. 



oesophagus leads from it into a nearly cylindrical, straight, 

 alimentary canal, which terminates in the anus, in the middle 

 line of the posterior extremity of the body. A mesentery is 

 attached to the whole length of the alimentary canal and 

 holds it in place. A nervous ring surrounds the oesophagus, 

 and posteriorly presents a ganglionic enlargement whence 

 nerves are given off to the body. The muscles are striated. 

 The sexes are distinct, and the males are usually much 

 smaller than the females. 



The testicle is an elongated sac which lies on the ventral 

 aspect of the intestine, and is connected anteriorly with two 

 vasa deferentia. These terminate on the fore-part of the 

 ventral aspect of the body, each having a saccular dilatation 

 which contains a very long, coiled, chitinous penis. In the 

 female, the ovary is also a large sac and the oviducts come 

 off from its anterior end, but the genital aperture is close to 

 the anus. 



The ova undergo their development in the ovary. The 

 embryos are oval, but taper to the posterior end. In the 

 middle line, in front, are three sharp protractile styles, of 

 which the middle is the longest. Two pairs of articulated 

 limbs are attached to the middle of the ventral aspect ; each 

 is terminated by a double hooked claw. The embryo of Lin- 

 guatulci thus resembles those of the Acarina^ on the one 

 hand, and those of such parasitic Crustacea as Aiichorella, 

 on the other. 



In the case of Pentastomum tcenioides, the embryos, in- 

 closed in their vitelline membranes, pass out of the bodies of 

 the dog or wolf, along with the nasal mucus. Taken into the 

 body along with the food of the hare or rabbit, they emerge 

 from the egg, penetrate the walls of the intestine, and lodge 

 themselves in the liver. Here they become encysted, grow, 

 and go through a series of changes of form, accompanied by 

 repeated ecdyses, until they pass into the state known as 

 Pentastomum denticulatum. If the flesh of the rodent con- 

 taining P. denticulatum is devoured by a dog, the parasite 

 passes into the frontal sinuses or maxillary antra of tho 

 latter, gradually takes on the form of P. tcenioides, and ac- 

 quires sexual organs. The parasitism of the Pentastomida^ 

 therefore, is very similar to that of the Cestoidea, 



Spiders and Mites abounded in the tertiary epoch, as 

 their remains, preserved in amber, show. Various Arthro- 

 gastra occur in the mesozoic formations, while Spiders and 



