ARENICOLA. 181 
Fig. 108.—LaTERAL Virw oF Front Enp or 
PoLtycorpDius. (After FRAIPONT.) 
Ciliated 
Peristomium. Pit. Prostomium, 
1 
Mouth. 
The sexual products escape by rupture of the body-wall. The 
following points in the development may be noted :— 
. Fertilisation external. 
. Total equal segmentation to form a dlastila. 
. Invagination to form gastrida and closure of the blastopore. 
. Elongation of the larva and invagination of anterior oesophagus 
and posterior hind-gut, forming mouth and anus. 
. Production of ¢vochophore, 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 ccelom, 
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. 
wm BWNHe 
II.—ARENICOLA. 
PHyYI.UM ANNULATA (page 237). 
Sus-PHYLUM ANNELIDA (page 238). 
Cass POLYCHATA (page 239). 
Arenicola marina is a worm, usually about 8 inches 
long, found very commonly burrowing in the sand between 
Hapits, ‘ide-marks. Its burrow is U-shaped, and from 
one to two feet in 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. 
