426 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 39. 



The internal anatomy of this seemingly simple thelycum is unex- 

 pectedly complex. While in a median longitudinal section the shelf 

 is seen to pass down into the general level without overhanging and 

 to be covered over by the scales with but a small simple cavity 

 between, j^et in a section to one side of the median line, fig. 7, we find 

 the shelf overhangs posteriorly a space which is continued both for- 

 ward and backward to end in branching chambers. In cross sections 

 these chambers (figs. 8, 9, and 10) are repeated right and left. In 



;^>:r'.V 



'.C^V 





'km 



lfi^,V>y^.:. 



^A^5. 



V-ift 









• / - /:.''r~c a: • 



;,''■.■-- 0:'^~ .%»',; 



Fig. s.— Cross section of anterior part op thelycum of PARAPENiEus constrictus, showing the 



SHELF (S) PENETRATED BY LATERAL CAVITIES RIGHT AND LEFT. 



fig. 8, which is across the anterior part of the shelf, the chambers are 

 two large separate cavities in the substance of the shelf. Each has 

 its horizontal and vertical portion, and each leads back in other 

 sections to the common narrow cavity between the shelf and the 

 scales. Again the section, fig, 9, through the posterior part of the 

 scales shows two large cavities hollowed out in the substance of the 

 scale plate and expanding as horizontal and vertical portions. These 



Fig. 9.— Cross section posterior to that shown in Fig. 8. 



chambers run forward to open into the common median cavity, as 

 shown in fig. 10, which is cut across the middle of the thelycum and 

 shows the two scales (Sc) overarching the shelf (S) ; the space between 

 the shelf and scales passes on each side as a tube to expand as 

 the chambers seen in fig. 9. The entire cavity into which the sperm 

 mass might be thrust in under the scales is thus seen to be somewhat 

 H -shaped, with the anterior horns converging and branched in the 



