71 
the sensory and nervous centre of the animal, directed for- 
wards during movement, as is already the case in Volvox, 
and constituting or forming part of the head, as in most 
Annelids, Molluscs, Arthropods and Vertebrates, and also in 
Balanoglossus. Sessile animals, however, often attach them- 
selves to their substratum with the animal pole which then 
loses its significance as a sensory and nervous centre. This 
is the case in Coelenterata, Echinodermata and Ascidia. In 
other forms again the same phenomenon is the consequence 
of the use of the prae-oral lobe as an organ to bore through 
the earth or the sand, as e.g. in earthworms, Gephyrea, 
Balanoglossus, where we also can speak of a praeoral lobe, 
and Amphioxus. 
In free living forms, including Vertebrates, the animal 
pole as a rule indicates the ante:ior end of the body, as it 
already does in Volvox and in pelagic blastulae and larvae. 
While tke four animal cells of the eight-celled stage, i.e. 
the animal half of the blastula, give rise to the prostomium, 
the whole segmented soma of Annelids, ecto-, ento- and 
mesoderm, grows out from the vegetative half of the blastula, 
the hyposphere or sub-umbrella of the larva. In Annelids, in 
Arthropods and in Vertebrates one or a number of segments 
of the soma form. together with the prostomium, the head. 
Head of Amphioxus. — In Amphioxus *) there is hardly 
any question of a similar process. Though in the adult stage 
as well as during development secondary modifications play 
a very great rôle, yet the original structure of the most 
primitive Chordate—the link between Annelids and Craniates, 
still missing in the first edition of this theory (1913) — may be 
clearly recognized in certain early stages oî development. 
The stomodaeum of the Annelid has grown out in a 
backward direction and has become the medullary tube. 
The mouth of the Annelid, situated ventrally just behind 
the limit of prostomium and first segment (peristomium), 
is found again as the neuropore of Amphioxus on the 
corresponding place, viz: dorsally, at the boundary of 
prostomium and soma, just in front of the first or “mandi- 
bular” mesodermic segment. In consequence of the burrowing 
mode of life the prae-oral lobe has an equally dull 
appearance as e.g in Lumbricus. The eyes, formerly situated 
1) The reader is invit-d to compare the following descrip'ions 
with plate 1. 
