390 INVEETEBEATA CHAP. 



Mesoderm cells in front of this multiply and form a string leading 

 from the aboral thickening to the ventral surface ; this string is the 

 rudiment of the main dorsal muscle of the larva. Shortly afterwards 

 an ectodermic invagination is formed in the posterior part of the oral 

 face. This is the proctodaeum, the rudiment of the anus and of the 

 larval intestine. It grows in length and joins the stomach, and the 

 latter opens into the stomodaeum, in which cilia become developed, 

 and so the definite alimentary canal is completed and feeding begins. 

 A delicate bivalve shell is secreted by the larva ; each valve is tri- 

 angular, and the apical organ protrudes between the apices of the 

 valves, whilst their bases flank the corona. 



Soon afterwards the ring of ciliated ectoderm which we have 

 termed the corona, and which we may compare to the prototroch of 

 the Trochophore larva, begins to exhibit modifications. In front of 

 the mouth a pair of transverse ridges grow inwards from it at right 

 angles to its course, and constitute a transverse band of cilia across 

 the ventral face of the larva. A pair of similar ridges also grow in- 

 wards from the oral band in front of the anus, and constitute a 

 second transverse band of ciliated ectoderm there. 



When the Cyphonautes is fully grown it possesses two other 

 organs : a so-called " piriform organ " in front of the mouth, con- 

 sisting of columnar ectoderm cells, and an "internal .sac," which 

 is an invagination of the ectoderm between mouth and anus. The 

 " piriform " organ arises as an ectodermal invagination, which becomes 

 almost shut off from the exterior, but remains connected therewith 

 by a narrow longitudinal slit, the cells lining which are covered by 

 powerful cilia. This slit is termed the vibratile cleft (v.cl, Fig. 314). 

 Though these ciliated cells afterwards meet those of the corona they 

 originate quite independently of it, and only subsequently come into 

 contact with it. The cells of the piriform .organ itself take on a 

 glandular appearance, and emit a secretion into its cavity. The 

 main muscle, alluded to above, leads from the apex and sides of the 

 piriform body to the apical organ, and then passes beneath this to 

 run down the posterior aspect of the larva to the most posterior cells 

 of the corona. 



In front of the ciliated groove which forms the opening into the 

 piriform organ there is a rounded ciliated pit which is delimited from 

 the groove by a blunt prominence. On the hinder aspect of this pit 

 there is a small group of cells which carry exceptionally long cilia 

 cilia which, moreover, are bent in a peculiar hook-like manner, and 

 which swing backwards and forwards in the middle line. These are 

 termed the vibratile plume. A special branch of the main dorsal 

 muscle pierces the glandular sac of the piriform organ and continues 

 its course to end at the base of the cells carrying this special " vibratile 

 plume." A strand of nerve fibres accompanies this muscular strand. 

 But the main mass of the muscle and nerve proceed downwards, 

 and whilst the nerve fibres become more numerous the muscle fibres 

 decrease in number. Many of the nerve fibres go to the ciliated cells 



