278 LARVAL FORMS. 



valent to those of the trunk. The embryonic peristomial region may, 

 on the other hand, be regarded as in a certain sense the first 

 segment. Its exact relations to the succeeding segments become 

 frequently more or less modified in the adult. The prae-oral region is 

 in most larvas bounded behind by the ciliated ring already described. 

 On the dorsal part of the prae-oral lobe in front of this ring are 

 placed the eyes, and from it there may spring a variable number of 

 processes which form antennas or cephalic tentacles. The number 

 and position of these latter are very variable. They appear as simple 

 processes, sometimes arising in pairs, and at other times alter- 

 nating 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- Ed wards, Claparede, etc. It arises 

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

 the prse-oral lobe containing a parenchyrnatous 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. 1 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 Serpulidse. 



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 prse-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, t) first appears, and then the left, and 

 subsequently the odd opercular tentacle 

 which covers the right original tentacle. 



FIG. 154. LAEVA OF SPIRORBIS. ^ ne third and fourth tentacles are formed 

 (From Alex. Agassiz.) successively on the two sides, and rapidly 



The first odd tentacle (t) is shewn become branched in the succeeding stages. 

 oil the right side. , 1T . , < .> 



Behind the pros-oral ciliated ring With reference to the sense organs 



is the large collar. 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 



