190 



THE EYE IN EVOLUTION 



Nematode, 

 Ascaris 



Arenirola 



Hcemadipsa 



Luinbricus 



Geonemertes, differ from the usual type. In these the pigmented 

 epithehum forms a complete circle within which is a mass of refractile 

 material ; the nuclei of the sensory cells are arranged outside the circle 

 of pigment and their distal terminations pass through it into the 

 central refractile mass (Schroder, 1918). 



In the NEMATODES, the majority of which are endoparasitic, sense organs 

 are Hmited to papillae on the lips ; in the free-living sexual state, however, 

 rudimentary eyes may exist, consisting of a lens-like cuticular body resting on a 

 cup of pigmented cells (Steiner, 1916 ; Hilton, 1921 ; Schulz, 1931). 



SEGMENTED WOBMS (ANNELIDS) 



The segmented worms exhibit much diversity in habit and 

 structure but their essential characteristics are segmentation of the 

 body with paired appendages on each segment and a closed vascular 

 system. Annelids are found both in marine and fresh water and on 

 land, and in the entire phylum more than 6,500 species are known. 

 These are divided into 4 classes, the first two of which are provided with 

 chitinous bristles or setae for locomotion. 



1. OLiGOCH^TES (with few setne), hermaphroditic creatures, essentially 

 terrestrial in habit, typified in the common earthworm, Lumhricus terrestris, or 

 the tiny aquatic mud-worms living in brooks or between tide-marks. 



2. polycHjEtes (with many setae), essentially marine in habit ; in them 

 the sexes are separate. Two types exist, distinguished by their habits. The 

 more active forms (errantia) are typified in the common lob-worm, Arenicola 

 marina, found burrowing in sandy beaches, or the freely-swimming types, such 

 as the rag-worm. Nereis. The sedentary forms (sedentaria) are tubicolous in 

 habit leading a sluggish life within tubes, limy, sandy or gelatinous ; as an 

 adaptive characteristic the tentacles, gills and sensory organs are aggregated in 

 the anterior part of the woi-m which protrudes from the tube. 



3. ARCHiANNELiDS Comprise a small and anomalous class of simple marine 

 worms with juvenile characteristics and without seta?, freely swimming or 

 burrowing in sand and gravel. 



4. HiRUDiNES or LEECHES form a highly specialized and much modified 

 class, most of which live in fresh water in ponds or sluggish streams although 

 a few are marine and others (the wiry land-leeches of the Far Eastern jungles, 

 Hcemadi])sa) are terrestrial, living in inoist places. In habit they are greedily 

 suctorial, sucking the blood of fishes, amphibians or other victims. 



Eyes are usually lacking in the oligoch^tes ; of those possessing 

 visual organs, the most typical example is the earthworm, Lumhricus 

 terrestris. Its unicellular light-sensitive organs distributed in the 

 epithelium and aggregated around subepithelial nerves have already 

 ])een fully described ^ (Figs. 86, 88). These visual elements are situated 

 \N iiere they are of the greatest biological value, being concentrated at 

 til; vo extremities, particularly the anterior. 



131. 



