ONTOGENY OF THE ANNULUS VENTRALIS. 123 



ratory. In large adults, 100 mm. long, the annulus in this 

 species has the appearance outlined in Fig. I, which is enlarged 

 about twelve diameters. It is a transversely elongated plate, 

 part of the shell, with a central depressed area bounded behind 

 by a cross ridge and in front by two high tubercles or tuberosities. 

 Across the depressed area runs a zigzag line which is in reality a 

 closed suture whence a slit leads inward to a curved tube repre- 

 sented by the thick shaded line. The suture and the curved 

 tube both open out on one side into the depressed area by an 

 orifice partly under one of the tuberosities. The walls of the tube 

 are thick chitinous continuations of the shell, as indicated by 

 the broken lines. Underlying this chitinous mass is the epi- 

 dermis which forms it and which was found to be folded in as a 

 bent groove. A comparative study of annuli in several species 

 showed that while the external sculpturing is various, the presence 

 of a curved epidermal groove is constant and that, morpho- 

 logically, this sperm receptacle is a bent epidermal pocket lined 

 by chitin and opening to the exterior by a more or less closed slit. 

 The position of the annulus, as seen in Fig. 2 which is enlarged 

 one and a half diameters, is on the ventral surface of the thorax 



FIG. 2. FIG. 3. 



between the bases of the fourth pair of legs. The sternal surface 

 behind it is elevated as a rounded knob that may be of importance 

 in discharging the receptacle. Projecting forward towards this 

 are the short pleopods of the first abdominal somite. Anteriorly, 

 on the bases of the third pair of legs, are the large elliptical open- 

 ings of the oviducts whence the eggs when laid pass back over the 

 annulus. 



