5D4 WILLIAM BATESON. 
(1) The animal is ciliated and inhabits muddy sand. 
(2) The preoral lobe is enormously developed. 
(3) The notochord arises at the anterior end of the hypo- 
blast and grows forwards. 
(4) The origin of the central nervous system consists in the 
delamination of a solid cord of epiblast in the dorsal middle 
line of the middle third ; this, by invagination of its two ends, 
is afterwards extended as a tube in both directions. 
Other collections of nerve-fibre are afterwards deposited in 
various parts of the body, and finally a general network of 
nerve-fibre occurs at the base of all the skin of the body, 
especially in the line of the gill-slits. 
(5) The mouth originally faces ventralwards, but comes 
afterwards to open forwards, being not a sucking but a dig- 
ging mouth. 
(6) The gill-slits for along time are only one pair, but 
subsequently are repeated in pairs, increasing in number with 
increase in the size of the body. 
(7) The mesoblast arises as one unpaired pouch, followed by 
two pairs of pouches. 
(8) The blood-system is entirely peculiar, consisting of an 
anterior heart and a dorsal and ventral vessel, and in B. 
minutus of two lateral vessels in the intestinal region. The 
two former are united by a plexus of trunks, which are placed 
under the skin and below the walls of the gut. 
(9) The generative organs are repeated through a large part 
of the body; in the branchial region more or less following the 
repetition of the gill-slits. 
(10) Of the excretory system little can be affirmed. The 
cells of the mesoblast appear to have a power of forming 
concretions, probably excretory, in their substance, and 
then throwing them into the body cavity. Here they 
form small aggregations. A large gland (containing a 
plexus of vessels), apparently performing their function, 
exists in the proboscis cavity attached to the end of the 
notochord. 
From the proboscis cavity opens an asymmetrical ciliated 
