cxxxiv Introduction. 



and Nemaioidea, the muscular system does not possess the external 

 circular layer so universal in other Vermes, its presence being ren- 

 dered superfluous by the chitinization of the cuticular layer of the 

 body walls. This chitinous deposit is secreted by a subcuticular 

 'granular layer/ and as in Arthropoda it is frequently changed 

 by moulting during growth and metamorphosis. In many cases 

 the muscular envelope is divided into four bands by the median and 

 lateral lines. In the Nematoids, bladder-like processes are some- 

 times given off from the muscle-cells, in such abundance and pro- 

 portions as to fill up a large part of what in the entire class is 

 ordinarily a large perivisceral cavity ; and a second set of processes 

 given off from them make up the internal ' transverse muscles ' of 

 authors. In the CTiaetognatha the muscular fibres are very defi- 

 nitely marked with transverse striae. The digestive tract is 

 entirely absent in the Acanthocephali, and the short oesophagus 

 is said to open into the general cavity of the body in Gordius, and 

 into a simple coecal sac in Mermis. In the Nematoidea, whether 

 parasitic or free, a digestive tract is always present and always 

 proctuchous ; the anus being situated some distance anteriorly 

 to the caudal extremity in the free species, but terminally or sub- 

 terminally in the parasitic. The oral opening is always placed 

 terminally at the front of the body, a point of some consequence 

 in differentiating certain forms, such as Ichthydium and Chaeto- 

 notus, which have sometimes been referred to the Turbellaria, 

 and sometimes to the Rotifera, as well as to the Nematoidea. The 

 pharynx may be armed with horny plates or teeth, and the oeso- 

 phagus is often highly muscular, and the remainder of the digestive 

 tract simple, and of a uniform calibre. There is no blood-vascular 

 system as distin-^t from the perivisceral cavity, nor any respiratory 

 organs. The water- vascular depuratory system has, in compen- 

 sation for the absence of special respiratory structures, and, in the 

 parasitic Nematelminthes, in correlation also with their habits, a 

 very considerable development. In the Nematoidea it opens exter- 

 nally by an azygos ventral pore, and in some cases by two sym- 

 metrically placed lateral pores, both of which orifices are situated 

 in the anterior portion of the body ; in the Acanthocephali, the 

 homologous system appears to have no external orifice, tliough its 

 cutaneous ramifications are very extensive ; and it is possible, there- 

 fore, that this system may be in this order circulatory and nutri- 



