ARENICOLA. 



Fig. io8. — Lateral Vif.w of Front End of 

 PoLYGORDius. (After Fraipont.) 



Ciliated 



Peristomiuni. Pit. Prostomium. 



Mouth. 



The sexual products escape by rupture of the body-wall. The 

 following points in the development may be noted : — 



1. Fertilisation external. 



2. Total equal segmentation to form a blastitla. 



3. Invagination to form gastrula and closure of the blastopore. 



4. Elongation of the larva and invagination of anterior oesophagus 



and posterior hind-gut, forming mouth and anus. 



5. Production of trochophore, with three bands of cilia, pre-oral, 



post-oral and peri-anal, paired larval "kidneys" consisting 

 of branching blind tubes opening externally, an apical plate, 

 with pigment spot, and mesoblastic pole-cells laterally to the 

 hind-gut. 



6. Elongation and growth of the posterior region to form the body 



(except prostomium and peristomium) of the worm. Splitting 

 and segmentation of the mesoblastic bands to form the coelom, 

 whilst the walls form muscles. Formation of nerve-cord from 

 epiblast cells. 



7. Loss of ciliated bands and pelagic habit. Growth of prostomial 



tentacles and nephridia. The young worm assumes the creeping 

 burrowing habit of adult. 



II.— ARENICOLA. 



Phylum Annulata (page 237). 



Sub Phylum . . . . Annelida (page 238). 

 Class . . . . . Polych^eta (page 239). 



Arenicola marina is a worm, usually about 8 inches 

 long, found very commonly burrowing in the sand between 



_ ... tide-marks. Its burrow is U-shaped, and from 

 ' one to two feet m depth. At one end the sand 

 is thrown up at the surface in small coils or casts which 

 have been ejected from the animal, whilst at the other end 

 is a conical depression in the sand, below which rests the 

 head of the worm. The burrow is lined with a mucous layer 

 secreted by the skin. 



