lb 2 Introductio7i to Animal Morphology. 



Cerebratulus). The large nerve ganglia are joined by a 

 double commissure, and the lateral cords are thick. The 

 eyes are seldom lens-bearing (Polia coronata, Nemertes 

 Antonina, Tetrastemma) ; sometimes two long series of pig- 

 ment specks and nerves exist, as in Borlasia splendens. There 

 is usually a closed perivisceral cavity, except in the few fresh- 

 water forms. A separate circulatory system exists, with 

 currents, often oscillating to and fro. The straight intestine 

 has lateral cseca attached to the body wall by filamentary 

 mesenteries, which also support the sexual pouches. When 

 the latter are distended with eggs, they displace the other 

 viscera. Tetrastemma obscurum and Prosorhochmus Cla- 

 paredii are viviparous. They are united to the last order by 

 Microstomidae and Dinophilidse, and to Trematoda by 

 Prorhynchus. In some there is a larval stage (Pilidium — see 

 above, p. i6o*). 



This order is divisible into two sub-orders : — i. Arhagea — 

 with rudimental or no cephalic grooves (p. 158). This in- 

 cludes two families : — i. Borlasidae — with a simple, un-lobed 

 head, and two (Cephalothrix), four (ffirstedia), or many eyes 

 (Ommatoplea, Polystemma), or eyeless, with a terminal 

 (Borlasia) or sub-terminal proboscis (Valencinia). Prorhyn- 

 chus has a very short proboscis. 2. ChlamidocephalidEe — 

 head two-lobed, divided by a shallow or deep notch (Col- 

 pocephalus, Chlamidocephalus). The lobes may be long, 

 again lobed (Lobilabrum). 



Sub-order 2. Rhagophora — with head grooves, including 

 three families. 3. IMonorhagea — with one transverse groove, 

 eyeless (Tubulanus), or with two rows of eyes (Micruraf ), or 



* In the larvae of Echinodermata and Vermes, three types may be dis- 

 criminated — 1st, that in which a ciliary girdle divides the surface into an 

 anal and an oral field (Brj'ozoa, Gephyrea, Holothuria, Echinoidea) ; 

 2nd, that in which two ciliary zones divide the surface into three regions, 

 an oral, an anal, and a teiTninal (Asteriadse, some Chaetopoda, Fig. 22, B) ; 

 3rd, that in which one ciliary course separates the single oro-anal field 

 from an imperforate terminal one (Pilidium, Chaetopoda, Rotatoria, 

 Fig. 22, C). The second and third are derived from the first. 



t Micrara has a terminal, attenuated, and contractile style, a prolon- 

 gation of the body wall, ciliated externally. 



