primitive lungs 是指什么？

Air-breathing fish are a reminder of the evolutionary path from which land animals developed. Gas exchange may occur either through the well-perfused walls of gas bladders or in more specialized structures, regarded as primitive lungs, although the distinction between these is rather arbitrary. Of extant species the lungfish, as the name suggests, has the most structurally advanced gas-exchange system, with its lungs containing complex ridges of cartilaginous parenchyma covered in respiratory epithelium which closely resembles the alveolar epithelium of mammals (page 10).

能呼吸空气的鱼是水生动物进化为陆生动物的一个例子。它们的气体交换可能发生在灌注良好的气囊壁或被认为是原始肺的一种更特殊结构中，但气囊壁和原始肺区别的认定相当随意。顾名思义，肺鱼在现存的鱼种中拥有最先进的气体交换系统，它的肺含有复杂的、覆盖着呼吸上皮的脊状软骨，肺鱼的呼吸上皮非常类似哺乳动物的肺泡上皮（第10页）。

primitive lungs 只是指“more specialized structures”，不是指“the well-perfused walls of gas bladders or in more specialized structures”吧？否则没法比较了

谢谢