474 



INVERTEBRATE MOBPHOLOOT. 



various ommatidia. 



tPS' 



'! I 



._Rh 



Pg* 



T 



/ 



Fig. 317. — Ommatidium 

 OP Eye of Musca (after 



HlCKSON). 



c = ciystalline cone. 

 CO = coruea. 

 Pg = pigment cells. 

 r = retinula-cells. 

 Rh = rhabdom. 

 2' = trachea. 

 tv = tracheal dilatation. 



Other sense-organs occur in the various 

 groups, but may more satisfactorily be 

 considered in the special descriptions 

 of these groups. 



True nephridia similar to those of 

 the Annelids occur in Peripatus, but in 

 the Myriapods and Insects they are en- 

 tirely wanting, their place as excretory 

 organs being taken by the Malpighian 

 tubules. The Tracheata are bisexual, 

 the reproductive organs being typically 

 paired and opening to the exterior by 

 ducts, which may unite before reaching 

 the genital orifice. Accessory struc- 

 tures, such as a bursa copulatrix for 

 the reception of the penis and a recep- 

 taculum seminis occur in the female, and 

 vesiculsB seminales and accessory glands 

 in the male. The region of the body- 

 wall in the vicinity of the reproductive 

 orifice is in the Insects frequently in- 

 vaginated, adding a still greater com- 

 plication, and furthermore the terminal 

 portion of the duct in the male is 

 frequently capable of being evaginated 

 and thus serving as a penis, while 

 integumentary elevations or processes 

 of the last abdominal segment form 

 ovipositors in the females. 



I. Class Protracheata. 



This interesting group contains but a 

 single genus, Peripatus, which has, how- 

 ever, a wide distribution, species being 

 found in the West Indies and South America, at the Cape of 

 Good Hope and in New Zealand, thus indicating an original 

 wide distribution of the genus which has become extinct 

 except in these few widely-separated regions. 



Peripatus is an elongated cylindrical form, measuring 



m 



