2 1 8 hitrodudion to A nimal Morphology. 



wreaths and a ciliated ventral surface. From these larvae the 

 head becomes differentiated with its two segments ; then the 

 tail, the cilia are lost, and intermediate segments form by 

 gemmation, Spirorbis and Pileolaria have brood-pouches- 

 wherein the embryos pass their early stages of development. 

 Some retrograde in growth, e.g., many Cephalobranchs have 

 eyes and otocysts, which are lost afterwards. 



About 1500 species are known, none of which are true 

 parasites, though some are commensals ; thus the larvae of 

 Alciope live in Cydippe, Lepidonota cirrhulae in the tubes of 

 Chaetopterus insignis. Nereis fucata in shells occupied by 

 Paguri, and a little Amphinome in Pentelasmis anatifera. 

 They are mostly marine, littoral, a few freshwater or terri- 

 colous. They are divisible into four orders : — 



I. Oligochaeta ((7/7/(5£?, Scoleina, Gegefibaur) — mostly terres- 

 trial or freshwater, with few (2-8) rudimental bristles in each 

 cluster ; no tentacles, gills, nor cirri ; mostly hermaphrodite. 

 This includes the following families: — i. Ichthydiidae — un- 

 segmented, or with segments only indicated by bristly and 

 ciliary wreaths ; head ciliated. Ichthydium is not bristled, 

 while Dasydytes is bristle clad, with a simple truncate tail, 

 and Chaetonotus with a forked tail. Taphrocampa, allied to 

 these, has a mastax like a rotifer. 2. Naididae — segments 

 few ; skin thin ; head lobes united, often prolonged ; bristles 

 in two rows ; eggs simple, single, large ; hard-shelled in 

 Aelostoma. Aulophorus secretes a tube which it carries 

 about ; its upper bristles are hair-like, the lower stiff. 

 Mesopachys has all the bristles hair-like. Nais has its under 

 row of bristles fixed. 3. Enchytraeidae — bristles two-rowed, 

 equal, awl-shaped (Enchytraeus), or hook-like (Parthenope) ; 

 mouth segments united ; blood colorless ; eggs large, singly 

 included in capsules. 4. Tubificidae — mouth segments united, 

 often lengthened ; skin transparent ; four rows of simple 

 (Enaxes) or forked (Saenuris) bristles ; eggs large, without 

 albumen, several in one capsule. 5. Lumbricidae — cylindrical, 

 many-jointed, with a clitellum (except in Helodrilus and 

 Criodrilus) ; prostomium free (except in Criodrilus) ; skin 

 opaque ; head lobes conical ; bristles hook-like, two or more 

 rowed, or over the whole surface (Perichaeta) ; eggs many,. 



