336 LARVAL FORMS. 



other times alternating on the two sides. There is frequently a 

 median unpaired tentacle. 



The development of the median tentacle in Terebella, where there is in 

 the adult a great number of similar tentacles, is sufficiently remarkable to 

 deserve special notice ; vide Milne-Edwards, Claparede, etc. It arises long 

 before any of the other tentacles as a single anterior prolongation of the 

 prae-oral lobe containing a parenchymatous cavity, which communicates 

 freely with the general perivisceral cavity. It soon becomes partially con- 

 stricted off at its base from the procephalic lobe, but continues to grow 

 till it becomes fully half as long as the remainder of the body. A very 

 characteristic figure of the larva at this stage is given by Claparede and 

 Metschnikoff, PI. xvn., Fig. i E. It now strikingly resembles the larval 

 proboscis of Balanoglossus, and it is not easy to avoid the conclusion that 

 they are homologous structures. 



Another peculiar cephalic structure which deserves notice is the gill 

 apparatus of the Serpulidae. 



In Dasychone (Sabella) the gill apparatus arises (Claparede and 

 Metschnikoff, No. 336) as a pair of membranous wing-like organs on the 

 dorsal side of the prae-oral lobe immediately in front of the ciliated ring. 

 Each subsequently becomes divided into 

 two rays, and new rays then begin to sprout 

 on the ventral side of the two pairs already 

 present. A cartilaginous axis soon becomes 

 formed in these rays, and after this is 

 formed fresh rays sprout irregularly from 

 the cartilaginous skeleton. 



In Spirorbis spirillum as observed by 

 Alex. Agassiz, the right gill-tentacle (fig. 

 154, /) first appears, and then the left, and 

 subsequently the odd opercular tentacle 

 which covers the right original tentacle. FIG. 154. LARVA OF SPIROR- 



The third and fourth tentacles are formed BIS. (From Alex. Agassiz.) 

 successively on the two sides, and rapidly The first odd tentacle (t) is shewn 



become branched in the succeeding stages. on e j ig , ht * ide ' 



Behind the prae-oral ciliated ring 



TTT..I r ,1 is the large collar. 



With reference to the sense 



organs it may be noted that the eyes, or at any rate the cephalic 

 pigment spots, are generally more numerous in the embryo than 

 in the adult, and that they are usually present in the larvae of 

 the Sedentaria, though absent in the adults of these forms. The 

 Sedentaria thus pass through a larval stage in which they 

 resemble the Errantia. 



Paired auditory vesicles of a provisional character have been 

 found on the ventral side of the body, in the fourth segment 



