50 SUMMARY OF CURRENT RESEARCHES RELATING TO 



twelve single chromosomes ; the female pronucleus has six single chromo- 

 somes. In a rather complex way it comes about that half of the sperma- 

 tozoa have six chromosomes and half have only five. As might be 

 expected, there are two kinds of embryos in the uterus — with twelve and 

 with eleven chromosomes in the germ-cells (twenty-four and twenty-two 

 in the somatic cells, for each of the so-called single chromosomes falls 

 into two). Further study will show whether the embryos with eleven 

 chromosomes become males, and those with twelve become females. 



Platyhelinintlies. 



Study of Gyrocotyle.* — Edna Earl "Watson has made a careful 

 study of this interesting Cestode, which is primarily monozoic (like 

 Amphilincb). It seems to throw^ light on the probable pedigree of 

 Cestodes — from a Turbellarian stock. The author discusses the occur- 

 rence of the animal in the spiral valve of some Chimseridse, its habits, 

 its structure, its species, and its affinities. 



The genus is represented by four species : Gyrocotyle nigosa, 0. urna, 

 G. nigrosetosa, and G-fimhriata, which are distinguished by many char- 

 acteristics. Nothing is known of any intermediate host, and reported 

 occurrences of G. ritgosa in bivalves are probal)ly accidental. 



The functional orientation of G. fimhriata directs the acetabulum 

 anteriorly, the rosette posteriorly. The worm is capable of definitely 

 directed locomotion, and is very active under favourable conditions. 

 The exploring function of the acetabulum is strongly in evidence ; it 

 never serves for attachment. The posterior rosette never functions as 

 an organ of attachment. " This functional orientation is borne out by 

 evidence from the structure of the central nervous system, and by the 

 presence on the margin of invagination of the acetabulum of a pair 

 each of sensory pits and sensory papillaj, abundantly innervated by 

 heavy branches from the central nervous system." 



The acetabular portion of the nervous system is developed in con- 

 nexion with the acetabular sense-organs and with the power of locomo- 

 tion in a definite direction. It corresponds to the brain of Turbellarians 

 and to the anterior ganglionic commissure in Trematoda. The rosette 

 portion of the nervous system is developed in connexion with the 

 development of a powerful posterior organ of attachment, and is com- 

 parable to a posterior ring-commissure in tlie posterior sucker of a 

 heterocotylean Trematode. The rosette of Ggrocoiyle is in structure 

 and function a true scolex, as its innervation shows. 



It is proposed to regard the Cestode scolex as a posteriorly situated 

 organ of attachment, the " neck " or growing region as the antepenult 

 region, corresponding to the antepenult segment in Annelids, and the 

 proglottis as the intermediate region of a body from which the anterior 

 extremity has completely disappeared. 



The limiting membrane of Gyrocotyle consists of a surface layer of 

 delicate fibres in a homogeneous matrix, and immediately beneath this 

 a layer of transverse and a layer of longitudinal muscle-fibres, nou- 

 nncleated. These are connected by fine processes with a layer of large 



* Univ. Califoruian Publications (Zool.) vi. (1911) pp. 353-468 (16 pis.). 



