136 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



is exhibited by the anatomy of Appendicularia and by the ontogeny of 

 Amphioxus, he assumes that the simple Ascidians themselves are more 

 primitive than the Compound Ascidians, but his reasoning seems to me 

 to rest upon an erroneous conception of the value and significance of the 

 evidence from embryology. 



The resemblance between the larvas of the Simple Ascidians and the 

 primitive chordata is unquestionably phylogenetic, but it does not by 

 any means follow that the adult animals are older than the Compound 

 Ascidians, and Pyrosoma, Salpa, and Doliolum. The locomotor chordate 

 tadpole larva serves to distribute the species and to establish new sessile 

 animals at a distance from their fixed parents, and these parents might 

 attain to almost any degree of evolution without losing this phylogenetic 

 larval stage so long as its retention continued to be important. 



There is no reason for believing that an adult which has retained it 

 is itself primitive, and the great size, complexity and independent indi- 

 viduality of the Simple Ascidians seem to be marks of great specialization 

 rather than of low rank. The most characteristic and dominant organs 

 of a Tunicate are the branchial sac and the atrial system, and these 

 structures are peculiarly complex and highly evolved in the Simple 

 Ascidians. 



The leathery, lumpish "sea-squirts" are much less attractive than 

 the delicate, transparent, pelagic Tunicates, whose whole organization is 

 at once seen to be beautifully adapted to the conditions of their life, and 

 it is natural to assume that these latter are the most improved and 

 modern forms, and that the Simple Ascidians are much older, but the 

 muscular mantle and richly ciliated pharynx of the Simple Ascidians are 

 well adapted for protection and nutrition, and the number and diversity 

 of the families and genera and species of Simple Ascidians, and their 

 abundance and wide distribution, prove that their organization has been 

 brought into very complete harmony with their conditions of life, and 

 that, in this respect, they are highly specialized. 



The colonial Ascidians spread by budding ; and the accidents which 

 break up the colonies scatter the fragments and distribute the species. 

 Small colonies of Perophora and Botryllus are often taken in the tow-net 

 far from land, and the ancestral locomotor stage is much less essential to 

 these animals than to the Simple Ascidians. 



The adult Pyrosoma, Doliolum, and Salpa are themselves specialized 

 for locomotion, and as they are not dependent upon local conditions, but 

 are at home all over the ocean, they do not need locomotor larvaB, while 



