522 COMPARATIVE ANATOMY CHAP. 



Under the cuticular lens an epithelial layer lies as a continuation of the hypoder- 

 mis ; this represents the matrix of the lens and is called the vitreous body. Under 

 the vitreous body lies the layer of the retinulse. Each retinula is separated from its 

 neighbours by pigment cells and consists of 5 retinal cells. One rhabdomere belongs 

 to each retinal cell. The 5 rhabdomeres fuse in the axis of the retinula into one 

 rhabdome. 



IV. Glands Opening on the Outer Integument. 



These are very numerous in the Arachnoidea. Many of them are insufficiently 

 known, especially as far as their constitution and the physiological significance of 

 their secretions are concerned. We shall divide the different sorts of glands into two 

 principal groups : (1) such as open upon limbs, and (2) those whose ducts have no 

 apparent connection with limbs. 



1. Glands opening on Limbs. Among these we have in the first place the 

 spinning glands of the Araneidse, since the 2 or 3 pairs of spinning mammillae on 

 which they open are very probably rudimentary abdominal limbs. There are on each 

 side several variously constructed spinning glands, whose secretion, which hardens by 

 exposure to air, serves for forming the various sorts of webs. Among these different 

 pairs of glands there is one pair which only seems to occur in the female and to serve 

 for the spinning of the egg cocoon. Immediately in front of the anterior pair of the 

 spinning mammillae there is found in some Araneidce a paired glandular region, the 

 so-called Cribrellum, in which extremely numerous spinning glands open through 

 fine pores. The cribrellum perhaps also represents the last remains of another (4th) 

 abdominal pair of limbs. 



The spinning glands of the Araneidce are rightly reckoned among those variously 

 developed integumental glands (coxal glands, spinning glands, protrusible sacs), 

 which must be finally traced back to the segmental setiparous glands on the parapodia 

 of the Annelida. 



In Gibbocellum (Cyphophthalmidce) there are found on each side ventrally on the 

 2d abdominal segment 2 spinning mammillae, on which several spinning glands open. 



Glands which open on the 4 pairs of ambulatory legs (either on one or on several) 

 have often been observed. One large gland is found on each side of the cephalo- 

 thorax of the Scorpionidce. It emerges, at least in the embryo and in young animals, 

 on the coxal joint of the 3d pair of ambulatory legs. On the 1st and 2d pair at 

 the place where the glandular apertures lie in the 3d pair, there are bulgings which 

 suggest that there were once glandular apertures here also. The apertures of these 

 coxal glands are usually not demonstrable in adult animals. 



Similar glands having apertures on the coxae of the 3d pair of legs are found in 

 the Araneidce also, in the Tetrapneumones (Mygale, Atypus] as well as in some 

 Dipneumones. Here also it is often difficult to prove the existence of the outer 

 apertures in the adults, and here also slits may appear on other legs which correspond 

 in position with the glandular apertures of the 3d pair of legs. 



The fact that the coxal glands of the Scorpionidce and Araneidce are unmistakably 

 similar to the coxal glands of the Xiphosura in position (on the 5th pair of extremi- 

 ties), in structure, and in manner of opening, has been used as a further argument in 

 favour of the relationship of these latter with the Arachnoidea. 



In the Solpugidce and Phalangidce also coxal glands are said to occur, in these 

 cases on the bases of the last pair of legs. Their ducts have, however, not been 

 observed. The occurrence of coxal glands in the Acaridce has also been described. 

 In the Oribatidce, for example, they lie at the bases of the 2d pair of legs ; in the 

 Gamasidce, it appears, between the coxal muscles of all the legs. It is not yet known 

 if certain stigma-like pores near the bases of the 1st pair of legs of the Halacaridce 



