Ot?I(jIN AND STRUCTURE OF THE HEAD 13? 



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 consequei\ce 

 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 tie four animal ceils of the eight-celled stage, i e. 

 the animal half of the blastula. give rise to the prostomium, 

 the whole segmented somi of Annelids, ecto-, ento- and 

 mesoderm, grows out from the v-egetative half of the blastula, 

 the nyposphere or sub-umbrella of the larva. In Annelids, in 

 Arthropods and in Vertebrate^, 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 role, yet the original structure of the most 

 primitive Chordate — the link between Annelids and Cianiates, 

 still missing in the first edition of this theory (1913) — may be 

 clearly recognized in certain early stages of development. 

 The stomodaeum of the Annelid has grown out in a 

 backward direction and has become the medullary tube. 

 The mouth of ihe 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 Lumbrlcus. The eyes, formerly situated 



^) The reader is invited to compare the following descriptions 

 with plate I. 



