REPORT ON THE CRUSTACEA MACRURA. xlv 



its general form so constant, that when fully developed it is a good and invariable test 

 of specific character. 



In Lucifer I have been able to follow the development of this organ throughout its 

 various changes, from the very early form to the adult stage, with more completeness than 

 in any other genus. Thus, when the animal is but 5 mm. in length, the petasma is 

 present in the form of a bud; when the animal is 10 mm. long it has increased to 

 a considerable degree, and when it has reached the adult stage it is about 12 mm. 

 or half an inch long, the petasma is fully developed (PI. LXXX. fig. 1, and p.t.m.). 

 This organ may be seen varying (PI. LXXXII.) from the simplest condition to that of 

 the more perfect form through various genera; and is universally present in the adult as 

 a pair of large veds which become linked together in the median line and form a curtain 

 that stretches across the pleon between the limbs from one side to the other. 



Whether this veil is constantly present in the adult male, or only at certain periods 

 when required, I am not prepared to assert, but I am inclined to believe that the latter 

 is the more likely case, if we may judge from certain specimens of Aristeus in the collec- 

 tion. For instance, on PI. XLV. figs. 1, 2, andp are shown to exist in different degrees 

 of immaturity, although the animals from which they were taken are all fully grown and 

 well-developed males. 



When displayed to the full extent they form a curtain, as may be seen in Gennadas 

 parvus (PL LVII. fig. %>.p.), in which they extend from the pleopod on one side to that 

 on the other, being connected in the median bne by a series of small cincinnuli (fig. p."), 

 and attached on each side by a small pedicle to near the middle of the basisal joint of 

 the pleopoda. It may be also seen in a similar condition, but more irregular in form, 

 in Sergestes atlanticus (PI. LXIX. fig. p.p.). On PI. XXXIX. fig. 2" it is also shown 

 in a folded condition, in Pleoticus miilleri ; but perhaps its true value and importance 

 may best be understood from an examination of the structure as it is shown in 

 Pleoticus pectinatus (PI. XXXVIII. fig. p.p.)- Here the petasma is extended and 

 united in the median line. The curtain is seen to be attached by a small pedicle to 

 near the middle of the inner side of the basisal joint of the pleopod, its margins are 

 extended vertically to the central body of the curtain, which forms a groove in the 

 middle ; near the middle of the lateral margins are two excavated spaces, into which 

 the fifth pair of pereiopoda fall when required, and beyond them are two enlarged 

 lobes with hollow apices, the object of which is not clear, unless it be to assist in 

 clutching the fifth pair of pereiopoda when pressed down ; beyond the lobes the margins 

 are fringed with a series of long, curved, comb-like teeth, that are capable of securing 

 a hold on the female when the animals are brought into contact. 



From what we know of the means of fertdization in Lucifer, and from what Sars has 

 shown in his Report on the Schizopoda of the Challenger collection, 1 I think we may 



1 Zool. Chall. Exp., part sxxvii. p. 74. 



