vii ANNELIDA 163 



blastema at first meets the head blastema ventrally, then it 

 subsequently extends up at the sides of the embryo and envelops the 

 dorsal region. 



Long before this the mesodermic bands have developed withki 

 them a double series of coelomic cavities. These rapidly meet one 

 another above the nerve-cord and form the ventral sinus, they then 

 extend upwards at the sides of the alimentary canal and meet above 

 it forming the dorsal sinus. The septa, which at first would 

 naturally separate each coelomic sac from those anterior and posterior 

 to it on the same side, disappear in the region of the dorsal and 

 ventral sinuses, but laterally they thicken and form the great mass 

 of connective tissue which makes up the " parenchyma " of the leech, 

 and the cavities of the coelom between them are thus reduced to 

 narrow slits. 



We may note that in Clepsine (Glossiphonia] the dorsal angles of 

 the coelomic sacs, as they extend upwards towards the mid-dorsal line, 

 bear a row of cells called " cardioblasts." These cells when they 

 meet their partners (viz. when the right one meets the corresponding 

 left one) form the wall of the dorsal blood-vessel ; the lumen of the 

 vessel arises from the fusion of a string of vacuoles formed in the 

 cells. This dorsal blood-vessel is absent in the Gnathobdellidae. 



ALLIED FORMS 



In old classifications of Annelida a group termed Gephyrea 

 appears. This group is now totally broken up into three distinct 

 phyla, of the Echiuroidea, Priapuloidea, and Sipunculoidea. Of these 

 the last named possess a Trochophore larva and a most interesting 

 development, which will be dealt with later on, but which is most 

 assuredly not Annelidan in character. The development of the 

 Priapuloidea is entirely unknown. 



The Echiuroidea, however, possess a development which proves 

 them to be modified Polychaeta. The embryo starts free- swimming 

 life at a post-trochophoral stage comparable to that in which Nereis and 

 its allies begin their larval life. There is a well-marked trochophoral 

 " head," followed by a worm body with numerous closely adpressed 

 somites, which, however, do not develop parapodia. There are well- 

 developed protonephridia, and an apical plate and prototroch, a 

 ventral ciliated groove and larval muscles. The segments disappear 

 in the adult, which exhibits a simple undivided coelom, and a nerve- 

 cord with no ganglionic thickenings. 



AFFINITIES OF ANNELIDS 



We have now completed our survey of the various types of 

 Annelid development, and it will be seen that they are all easily to 

 be interpreted as abbreviations and simplifications of the long larval 

 history characteristic of Polygordius. The primitive nature of 



