148 INVERTEBRATA CHAP. 



underlying the prototroch, and another underlying the metatroch. 

 Apparently the radial nerves do not communicate with the plexus, 

 and in this respect the Trochophore resembles a Ctenophore, for there 

 is in the adult Ctenophore a plexus of nerve fibres under the 

 ectoderm which is quite distinct from the radial bands which underlie 

 the " ribs." 



As the larva grows the apical ciliated cells fall off and the spot 

 where they existed becomes a pit, soon overgrown from the sides by 

 thickenings of the ectoderm. These lateral masses (probably derived 

 from the Annelidan cross) form short thick pits from whose bases the 

 head tentacles arise (t, Fig. 108). The ganglion, however, persists 

 for some time, but it is soon overshadowed by two lateral masses of 

 nuclei which constitute the rudiment of the paired supra- oesophageal 

 ganglion of the adult. These masses lie beneath and to the outer 

 sides of the rudiments of the tentacles ; they probably arise from the 

 inner parts of the lateral arms of the " Molluscan " cross ; they enter 

 into connection with the two lateral nerves which ultimately form 

 the nerve collar of the adult. Behind the tentacle bases there appear 

 two shallow grooves : these acquire, later, stiff cilia and become the 

 ciliated pits (c.p, Fig. 108) of the adult. At the extreme outer 

 edges of the head blastema there appear two small pits lined by 

 pigmented cells. These are the eyes of the larva. 



Underneath the lateral nerves the lateral muscles are formed 

 later by the accumulation of mesenchyme cells. These connect with 

 the trunk blastema and persist, though they do not become 

 functional until the metamorphosis. A mid-dorsal muscle is formed 

 in the same way and is not functional during the larval period. 



These sub-epithelial muscles which are taken over into the adult, 

 are not to be confounded with purely larval muscles, mentioned above, 

 which traverse the blastocoele. These transitory larval muscles are, 

 (1) a pair of oesophageal retractors running from the head blastema 

 to the skin just above the mouth (oes.ret, Fig. 106) ; (2) a pair of 

 main retractors running from the head blastema underneath the 

 origin of the tentacles to the trunk blastema (ret, Fig. 108, B) ; (3) 

 a pair of dorsal "elevators" (elv, Fig. 112) arising from the dorsal 

 surface half-way between the apical plate and the prototroch ring, 

 and running downward to join the trunk blastema; (4) a plexus 

 of muscle fibres underlying the skin everywhere and crossing each 

 other in various directions ; and (5) two powerful ring muscles, one 

 underlying the prototroch and one the metatroch. 



Before the growth of the post-oral segmented region is quite 

 complete, the intermediate girdle cells intervening between the 

 region of the prototroch and the " head blastema," gradually degenerate 

 and disappear. They pass from a flattened to a columnar form, 

 their nuclei are dissolved, they assume a glassy appearance and then 

 are cut off either internally, when their remains are devoured by 

 amoebocytes, or externally ; and so the head blastema comes into 

 contact with the prototroch ring on its upper margin. A few of 



