28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



The xiphosurids copulate during the breeding season, but there is 

 no intromission of the sperm. The eggs are fertihzed outside the 

 body of the female and are then usually deposited in sand at the 

 bottom of the water, though the female of Tachyplcus gigas {Limu- 

 lus moluccanus) is said to carry them attached to the abdominal 

 appendages. 



V. EURYPTERIDA (GIGANTOSTRACA) 



These extinct chelicerates are usually included with the Xipho- 

 surida in the class Merostomata, but they have features characteristic 

 of the Arachnida, and a general appearance that gives them a resem- 

 blance to the scorpions. 



The body of an eurypterid contains i8 postoral somites (fig. lO A), 

 and ends with a telson {Tel) having the form of a large caudal spine 

 or plate. The two major parts of the body, prosoma and opisthosoma, 

 are separated between segments VI and VII, as in the Arachnida. The 

 first opisthosomal segment is represented on the dorsal surface of the 

 body by a small tergal plate (C, VII), but is not visible on the ventral 

 surface (A, B). The first live ventral plates of the opisthosoma 

 (A, B, VIII-XII) are said to bear gills on their upper surfaces, and 

 hence probably represent the appendages of these segments united 

 with the median sterna. The first ventral plate (A, Opl), since it 

 belongs to segment VIII, represents the operculum of Xiphosura. 

 It consists of two broad lateral parts separated by a median appen- 

 dicular process (oap) of various forms. This opercular process, by 

 comparison with the abdominal appendages of Xiphosura (fig. 9), 

 would appear to be formed, as suggested by Stormer (1934), of the 

 united telopodites of the opercular appendages. It has been supposed 

 to be a genital organ, but this idea seems improbable except in the 

 sense that the process may be a genital accessory of some kind. The 

 genital openings have not been discovered, but it is to be supposed 

 that they are concealed beneath the operculum. 



In some of the eurypterids the opisthosoma is subdivided into a 

 preabdomen and a postabdomen (fig. 10 B, C) by a difference in the 

 width of the segments and their apparent mobility. Such forms have 

 a resemblance to the scorpions (D), but it should be observed that 

 the division between the two parts of the opisthosoma occurs between 

 segments XIV and XV in the scorpions (D), while in the eurypterids 

 (B, C) it is between segments XIII and XIV. In each group, how- 

 ever, there is the same number of segments in the " tail ". Hence, it 

 is possible that the eurypterids have lost one of the segments of the 



