DESCRIPTION OF SPECIES 139 



difference in the ratio of stalk to body. This is confirmed by using the coefficient of difference (Mayr, 

 Linsley & Usinger, 1953) to determine how far this character overlaps in the two populations. The 

 level of distinctness of the populations is much below that conventionally recognized as subspecific. 1 

 It is therefore clear that Ascopera, as represented in the rich ' Discovery ' material, cannot be separated 

 into two species or even geographical forms. I therefore agree with van Name (1945) that A. peduncu- 

 lata is a synonym of A. gigantea. 



Branchial sac. The irregularity of the stigmata and transverse vessels (Text-fig. 61 A) has been 

 stressed in descriptions of Ascopera, but in some specimens, especially young ones, the typical 

 molgulid arrangement of spiral stigmata is readily seen (Text-fig. 61 B). There is also, in small 

 animals, less irregularity in the vessels, many of which are still transverse and unbranched. As in 



B 



Text-fig. 61. Ascopera gigantea Herdman (St. 371) : A, part of the branchial wall of a large specimen ; B, of a small specimen. 



other molgulids, too, the stigmata form infundibula, which rise into the branchial folds to form 

 pouches. Longitudinal bars have the following arrangement in two specimens of moderate size : 



(1) dorsal line— 5 (12) 3 (8) 7 (7) 4 (9) 6 (9) 8 (5) 6 (9) 3 endostyle. 



(2) dorsal line — 4 (9) 1 (16) 4 (14) 3 (11) 6 (13) 6 (7) 2 (8) 2 endostyle. 



Gut. Yellow glandular tissue on the surface of the stomach extends down each side of the intestine 

 for some distance (Text-fig. 62 A, gl.). 



Gonads (Text-fig. 62 B). The gonads are long sausage-shaped bodies, each with a central ovary 

 (ov.) extending the whole length of the gonad, and a series of testis lobes (t) arranged along the sides 

 of the ovary. One terminal oviduct (od.) is normally present, pointing towards the atrial siphon, but 



,„~. .... ,. .. ^.r. mean, — mean. TT , rr- ■ 



1 Coefficient of distinctness of two populations ^and5= 3 — -7-3 — — -3 — -j-j — — — '• Here the coefficient 



r r standard deviation. — standard deviation. 



of distinctness = 



•1 — o-6 



0-3742 + 0-3755 



= 0-667, which is much below the conventional level for subspecific distinctness (1-28). 



18-2 



