ARACHNIDA. 467 



of the apodal and acephalous worm ; the rudiments of the head, with 

 its eye-specks, and of the limbs, are manifested before the vitelline 

 mass is included by the abdominal walls or intestinal membrane: 

 the tripartite character of the insect — head, thorax, and abdomen — 

 is faintly and briefly indicated ; but, with the first indications of the 

 ocelli, trophi, and legs, the special arachnidal form is acquired. 



In certain aquatic Arachnida, Nymphon, Pycnogonum, e. g., there 

 is such a change of proportions of the body and limbs as might 

 deserve to be called a metamorphosis ; but even these first acquire the 

 Arachnidal type before retrograding to a lower parasitic form. Most 

 mites have only six legs when hatched, but they otherwise resemble the 

 adults, and after the ecdysis acquire the fourth pair of legs. The 

 young of Hydrachna are likewise at first hexapod, and they have 

 also a long and large snout, like a distinct head. With this they 

 pierce the bodies of water-insects, and pass their first stage in a 

 parasitic state, in which they have been called AchlysicB* 



The regeneration of the legs of the spider follows precisely the 

 same law as that which regulates their reproduction in the Crustacea. 

 If the limb be injured at the tarsus, tibia, or femur, it must first be 

 cast off at the coxo-femoral joint, before the process of reproduction 

 can commence, and this must be preceded by a moulting of the in- 

 tegument ; the new leg being at first of small size, but with all its 

 joints and appendages, and acquiring the full proportions at the 

 second moult. The palp, with the accessory generative organ, has 

 been reproduced in the male, and has proved to be efficient. 



Class ARACHNIDA. 



Head usually confluent with thorax, and provided with modified, 

 commonly mandibuliform, antennee : no wings ; four pairs of legs. 



Respiratory system, when present, tracheal, or pulmonary, or 

 both. 



Order Dermophysa. 



No distinct respiratory organs: respiration performed by the 

 common integument. Cephalothorax multiarticulate. 



Tarbigrada. Legs rudimentary ; abdomen wanting : herma- 

 phrodite. 

 Genera Macrohiotus, Emydium, Mihiesium. 

 NuLLiGRADA. Lcgs rudimentary ; abdomen long and slender : 

 hermaphrodite. 

 Genus Demodex. 



* CCXCI. p. 138. taf. 1. 

 H H 2 



