92 K. ZIMMERMANN. 



The flagellum of one of the niaxillipedes (probably of the 2nd) is ob- 

 viously of use in strengthening and accelerating the ex-current stream, 

 and its motion or stillness forms a good guide to the progress of the 

 breathing activities on either side of the body, in addition to the indica- 

 tion furnished by the motion of particles in the water. It was noticed 

 that both P. longicornis and P. platycheles, when at rest, almost in- 

 variably hold the chela of one side (a) further away from the body than 

 its fellow (/3) : subsequent observation, many times confirmed, revealed 

 the fact that 07ily from the exhalent aperture of the a side did an ex- 

 current stream proceed : apparently no respiratory stream was passing 

 through the branchial cavity of the ^ side, and certainly the flagellum 

 of that side was at rest. Supposing that the chela of the left side is held 

 away from the body : only the left flagellum is meanwhile at work, a 

 strong ex-current stream proceeds from the left side only, and the anten- 

 nules, at their water-testing work, are constantly directed towards the 

 riffht side of the animal (Plate 2, C and D). Meanwhile, on the right side 

 of the body issues no ex-current stream, but generally the right antenna 

 is kept in fairly constant motion, being swept from front to back, and 

 vice versa. In other words, there is apparent suspension of the respira- 

 tory function of the right side of the body, compensated by the re- 

 doubled activity of the sensory function of the same side. In a normal 

 captive Porcellana, the duration of this period of suspension is usually 

 about half an hour ; at the end of that time the animal becomes restless, 

 " fidgets," and ends by reversing the functions of the two sides of the 

 body, after a brief period of irregular movement of first one flagellum, 

 then the other, and occasionally even of both at once. It must be noticed 

 that in Porcellana there is no reversal of the current such as M. Bohn 

 observes {C.R. Ac. Sci., CXXV, 1897, p. 441, "Sur le renversement du 

 courant respiratoire chez les Decapodes ") in Carcinus, etc., and which 

 may serve to rest the muscles of the scaphognathite or to cleanse the 

 branchial cavity. Probably this resting of the muscles is at least as well 

 ensured by the alternate working of the organs of the two sides. M. 

 Bohn remarks that, in animals which have the reversal habit, the number 

 of reversals within a given time may be increased by placing the animal 

 in a toxic solution such as extract of Red Seaweed. Similar experiments 

 performed on Porcellana produced no reversal of the current, but had 

 the effect of causing the alternations in function of the two sides to become 

 rapid and irregular. Galathea exhibits no such alternation, and on placing 

 it in Red Seaweed extract there was detected distinct reversal of the 

 respiratory stream, although the backward current produced was but 



