132 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



The most characteristic peculiarities of the Salpa embryo are its 

 foetal development, and the presence of a placenta and foetal envelopes. 

 The Pyrosoma embryo does not present the slightest trace of these pecu- 

 liarities, nor is there any indication whatever in the life-history of Salpa 

 of the degeneration which is so remarkably manifested by the Pyrosoma 

 embryo, and we cannot believe that the life-history of either of them is 

 like that of their common ancestor. 



In fact the differences are so great that if the resemblance between 

 the embryos were the only thing which these animals have in common, 

 we might well doubt whether this alone is sufficient to prove their 

 affinity, but their whole organization testifies to their very close relation- 

 ship, and if we make a comparison, not between Salpa and Pyrosoma 

 alone, but between Pyrosoma on the one hand and Salpa, Doliolum and 

 Anchinia on the other, the anatomical resemblance is most impressive. 

 It is exhibited not only by the fundamental plan of their structure, but 

 also in superficial details. 



Thus the primary colony in Pyrosoma consists, as it does in Salpa 

 pinnata, Plate I, Fig. 2, of a wheel or rosette made up of a number of 

 zooids placed parallel to each other with their dorsal surfaces outwards, 

 their ventral surfaces towards the axis of the colony, and their mouths 

 all pointing the same way. 



It is also interesting to find in both Salpa and Pyrosoma peculiar 

 luminous organs with the same structure, and in positions which, while 

 not strictly homologous, are sufficiently alike to show that they have in 

 all probability been inherited from a common source. 



The solitary Salpa pinnata, Plate I, Fig. 6, has five pairs, and the 

 aggregated form of the same species, Plate I, Fig. 3, one pair of longi- 

 tudinal rod-like organs in the intermuscular spaces at the sides of the 

 dorsal surface. 



These organs, which were noted by Forskal, have given rise to much 

 speculation, although no one could examine a living Salpa pinnata with- 

 out discovering that they are intensely luminous. I am not aware that 

 this interesting fact has ever been recorded. They have been regarded 

 as ovaries (Meyer), or renal organs (H. Muller), but recent writers have 

 refrained from conjectures as to their function. 



My first living specimens of this species were examined on deck 

 under the full blaze of the noonday sun, but in spite of the bright day- 

 light my eye was instantly caught by these longitudinal rods which 

 glowed with a light of their own as the animals floated in a dish of 



