82 NOTES AND OBSERVATIONS ON BOARD H.M.S. RATTLESNAKE 



of the first or internal pair of antennae is much longer than the others, 

 and is slightly enlarged at its base. The enlargement contains a 

 clear vesicle {e), slightly enlarged anteriorly, but not communicating 

 by any fissure with the exterior. It is about ^f-Joth of an inch in 

 diameter. It contains a spherical strongly refracting otolithe about 

 YuVo-th of an inch in diameter, which does not present any vibrating 

 or rotating motion. We have here then Lucifer presenting an organ 

 precisely similar to the auditory sacs of the Mollusca, while Palcemon 

 offers a very interesting transition between this and the ordinary 

 crustacean form of acoustic organ as described by Farre, and there 

 can I think be very little doubt that the determination of the latter (as 

 regards the Macroura at least) was perfectly correct. 



Since writing the above I find that the auditory organ in Lucifei'- 

 has been recognized by M. Souleyet. All that he says about it is 

 contained in the following lines : — " Bei einigen See-krustenthieren 

 namentlich bei der Gattung Lucifer (Thompson) habe ich ganz 

 neuerdings an der Wurzel der innern Fiihler einen kleinen runden 

 glanzenden Korper entdeckt der mir dasselbe Organ (auditory organ) 

 zu seyn scheint." — Froriep's Notisen, 1843, p. 83. 



EXPLANATION OF PLATE XIV. [Plate 10] 



Fig. I. The line indicates the natural size of the animal in this and the following figure : 

 u, internal antennae ; b, external antennae ; c, basal lobe of external antennae ; 

 (/, eye ; f, otolithic sac. 



Fig. 2. Yitad oi PaliTino/i. Letters as in fig. i. 



Fig. 3. Internal antennae of Pala^jnon enlarged : a, spine ; b, auditory sac -j r, pigment- 

 granules ; if, curved line to which the hairs are attached ; e, otolithe ; f, antennal 

 nerve. 



II. On the Anatomy of the genus Tethya. 



The animal which forms the subject of the present communication 

 was found attached to rocks and stones, close to low water mark, upon 

 the shores (skirting one of the smaller bays of Sydney harbour) of the 

 beautiful grounds of my friend Mr. W. S. MacLeay.i 



MM. Milne-Edwards and Audouin (Ann. d. Sc. Nat. 1828, tom. 

 XV.) and Dr. Johnston (British Sponges and Lithophytes) are, so far 

 as I am aware, the only authors who give any detailed account of the 

 genus Tethya. 



Of the two species described by the latter, T. Lyncurium approaches 

 nearest to the present species ; the only difference being that while 



1 It is not necessary for me to speak of Mr. MacLeay's singular acquirements and acumen ; 

 but I cannot refrain from taking this opportunity of expressing my deep sense of the benefit I 

 have derived from his advice and assistance — always most readily offered. 



