276 



ANNELIDA. 



different viscera in the interior of the hody become very clearly de- 

 nned. 



(708.) The digestive apparatus is now distinctly perceptible : ante- 

 riorly it presents a kind of fleshy bulb (fig. 136, 4, p) ; then a short 

 cylindrical oesophagus, followed by a capacious ovoid stomach (r), the 

 contents of which appear to be still saturated with the coloured sub- 

 stance of the vitellus, and an intestine (s), which commences at about the 

 posterior third of the body. The glandular structures near the anterior 

 part of the animal now become apparent, and the subcutaneous muscles 

 clearly distinguishable ; still it is remarkable that, even in the most 

 transparent portions of the creature, no traces of a vascular system can 

 be detected. 



(709.) In the course of three or four days more, the cilia have com- 

 pletely disappeared from the surface of the body, which now presents 

 all the characters of one of the erratic Annelids, but in no respect re- 

 sembles the tubicolous genera to which the creature really belongs. The 

 young larva, in short, is furnished with a distinct head, an antennary 

 organ, eyes, and feet armed with subulate seta3 ; while the adult Tere- 

 bellse are acephalous, being destitute both of antenna and eyes, and 

 having feet provided with hook-like appendages. 



(710.) After the larva has been furnished with one or two additional 

 pairs of feet, the head begins to be changed in its shape (fig. 136, 5), a 



Fig. 136. 



Development of Terebella nebulosa, (After Milne-Edwards.) 



transverse constriction makes its appearance at a little distance in front 

 of the eyes, and its anterior lobe, which thus becomes distinctly defined, 

 is seen to be studded near its free margin with a series of stinging 

 vesicles, some of which are armed with little spine-like filaments. The 

 post-cephalic ciliated collar becomes at the same time much narrower, 



